Joist hangers "droop" when installed?
If the joist hanger is slightly wider than the joist ( joist, not stud )  then do not push it in towards the joist.

Fasten it parallel with one side up against the joist and a gap on the other side. Then fasten the gap side to the ledger board parallel so it does not bow at the bottom, ( do not push it in to the joist )    Before you fasten the gap side  to the joist put a filler shim in the gap between the joist and the hanger. A piece of thin plywood could work or two shims turned so the thin edge's are facing each other so they create a flat surface. ( This allows you to adjust the thickness of them by sliding them together or apart.   you may have to trim their length ) 

The connection of the hanger with ledger board is important as it is what is carrying the weight of the joist and the connection of the hanger to the joist help to keep it from pulling away from the ledger board. Use a longer screw or nail to account for the thickness of the shim.