Why is your beta swimming under the rocks and getting stuck?

Betta fish of nearly all kinds have thin, relatively flimsy fins to get through almost flowless lakes and ponds called Rice Patties. They spend a lot of time floating because it is a lot of work to swim about for them,(That's why its best to keep them in wide tanks instead of tall) as apposed to fish like koi and tetras, which have short, relatively sturdy fins use for propelling themselves through faster moving streams and rivers. In short, the betta fish is simply resting.

To make it simple, imagine you are under water and the only way to move about effectively is to quickly and rigorously flap your wrists/hands(not your entire arms) and only your wrists/hands until you reach your optimal location. Keep in mind feeding, fighting filter currents and accelerating. It's a LOT of work to be a betta fish!