Your caster assembly's insertion part is worn out and should be replaced, as an assembly. I think Fender sells replacement casters, as do many home-improvement-labyrinthine-warehouses or hardware stores.
Barring that, it looks like the "pin" portion that inserts into the cabinet can be spread out, so when reinserted, it may hold better. You can then hold a small block of wood, or a piece of 2X4, and give the caster a few "love taps" with a rubber mallet. I never use a hammer. I sometimes repair or reinstall casters that fall out of cabinets, due to movement or breakage in shipping, where I work (not amps, though).
Squier Classic Vibe Club #125; Fender Rumble Club #78.
Last edited by TheAnalogKid : 09-29-2013 at 05:50 PM.