Keeping all of your players involved and relevant is one of the things a Dungeon Master wrestles with. However, it's also something that is difficult to deal with. Magic items can do a lot to shift player focus. This is especially true in combat. However, a clever player can also make this happen in other areas of the game. There is also the question of whether it is fine to have some players that are focused more than others. What if you have a shy player? These are the ideas and aspects I hope to work through by putting into words.
What Can Shift Focus
I've already mentioned in passing that magic items can shift the focus to one player or character. However, it goes further than that. The location that something takes place in can also shift the focus. If you have a character that is native to the area, they will naturally be a focus for events in the story. The type of adventure can also do a lot to make some characters more of a focus than others. In D&D 5th edition they have what they call the 3 pillars of the game (combat, exploration, and role-play/interactions). Some classes are better and one than another in certain situations. You might find that the situation or campaign you came up with will naturally gravitate towards certain players or their characters. There is also the question of player experience or mastery of the rules. Some players can just work their skills and weaknesses in amazing and clever ways that makes them shine.
Should All Players Shine Equally?
When you first think about how players shine during the course of a game, the first thought that usually comes to mind is that they should all be equal. This isn't necessarily true. Or more precisely, what exactly does that mean? What I find is more accurate is that there is a certain amount that if you don't meet, your player(s) will feel unfulfilled. For some, this amount is more than others. They'll also get to shine in different moments and not all moments are perceived equally. It might also depend on the long view of things as well. They might want things to balance out in the long run, but for this session they might be fine taking a back seat. After all, last week's session was theirs.
Round Robin Focus
You could try to make a few of your party members the centre of focus for the session. The focus characters will be rotated between sessions and the end result is that everyone roughly gets the same amount of focus. It's kind of like when a TV show has episodes focusing on different characters. Of course, not everyone likes that kind of thing. It's also a bit different. You have the risk that someone might accidentally hijack the session by doing something clever or thinking outside the box. That might very well make sense, but it defeats the purpose of this approach.
You could instead design the situations players will end up in. Players will then decide how to approach the problem and when they do, decide who will shine in that moment. It feels quite naturally and lets the players decide who shines partially, but it still keeps the possibility of someone being overshadowed.
You could also combine the two ideas above, which is what more often happens naturally. You design situations for your players but you might try to target some of the strength of your players. There will be a combat encounter for your combat focused character, some kind of social interaction, maybe a history related element, and end it with a twist that involves one of your player characters.
Shy players are one of the harder ones to bring into the game. From my experience, they tend not to need as much time shining to feel like they contributed enough to the session. It can also be shining in a different way. You also don't really want to try and force them into the centre of attention, especially at the start. What I find typically works best is to let them get comfortable and choose when to be the centre of attention. At first, this might not seem to work. However, once they get used to the game and the people they are playing with I've usually seen them have no issue jumping in. I chose to say tend here because it really does depend on the person in question.
The early part can be a bit rough though. In this case, you probably don't want to have someone else stepping on their toes all the time. Having one rogue played by a shy person and another played by a social butterfly is often a recipe to have the shy person overshadowed (it isn't always the case, particularly if the two builds are very different with very different skill sets). Some overlap isn't so bad, since someone else can pick up some of the slack just in case. However, there needs to be some individuality and some cases where the shy character makes the most sense. It can also be a problem when the shy person feels more comfortable and finds themselves competing with someone else in the area that overlaps.
There are some things, magic items in particular, that could cause a player to be accidentally overshadowed. Easy access to healing magic items in particular can make things harder for the cleric. Likewise, certain magic items can make combat far easier for the player that has them. I usually weight magic items that don't have a limited number of uses very carefully to prevent this. Such items are fine, however, if your party is lacking something. If your 2 player party doesn't have a cleric but they have easy access to healing magic, there isn't a problem. Otherwise, care should be taken. It can especially be an issue at low levels. At that time even small bonuses make large differences and access to abilities that target large areas are disproportionately powerful. It's also a good idea to consider whether a magic item will make a particular class feature or character feature redundant. A sentient, and autonomous pair of lock picks can be a rogue's worst nightmare.