Since Shelled Amoebas are not filter feeders, they depend on their ability to hunt and seek out food. So, a Shelled Amoeba on its back will starve unless it finds some way to right itself. It has a way. It generates an air bubble on one side of the shell. This bubble raises that side so the creature now sits on its opposite side, like a coin on edge. It then extends and attaches its pseudopods to the solid bottom and pulls itself over. Ready to proceed once again as a hunter.
If you're not impressed by any of this, remember - these blobs are nothing more than one cell of "goop".