The Steam Deck is loaded with input options: four rear buttons, two triggers, thumbsticks, trackpads, a d-pad and ABXY buttons as well as a touchscreen. Unfortunately most games aren't set up to support these additional buttons so you'll have to remap their in-game controls yourself. This can be a tedious process for some games, but it's not impossible and can make your game play much more comfortable and enjoyable.
Many Steam Deck users report that the back buttons (L4, L5, R4, and R5) are difficult or awkward to press. This is likely due to their positioning and the mechanical nature of the button mechanism itself. Triggers, bumpers and thumbsticks are also prone to dropping damage as they protrude out from the rest of the console's profile.
Thankfully, these buttons can be easily remapped to almost anything, including system commands or even other inputs like the mouse cursor. This allows players to make use of Action Sets and other layout features that can change the way their games are played. It's something that the Xbox doesn't have, but I don't think it's overstating it to say it would be hugely beneficial for gamers who struggle with physical limitations.
If you're experiencing this issue, begin by visual inspecting the buttons and their internals for signs of damage or a loose connection. Cleaning with isopropyl alcohol and a bit of gentle elbow grease should remedy most issues. If all else fails, try performing a reimage of your Steam Deck using Valve's OS Recovery instructions. This will reset all user data but will leave your game installations intact. Be sure to follow general electrostatic discharge (ESD) safety procedures if reworking your console in this manner.