Possibles:
-Remote cable not pulling shift handle fully into forward gear.
-Shifter 'phase' not adjusted correctly.. with motor off, spin prop slowly while shifting between gears with shift handle. Forward and reverse should engage the clutch dog at the same distance from neutral (or bias a bit to forward).
-Forward gear 'ears' that mate to dog are worn too much.
-Bent shift fork, broken dog yoke..
Good luck!