Hi,
Is it possible that your taxi speed is too high?
Normal 90 degree turns should be initiated at 10 knots or less. When using full deflection make sure that your speed is 5 knots or less.
Always start turning VERY gently to give the aircraft a chance to change momentum from forward movement into the turn. IRL we initially used only a slight pressure and slowly increased that pressure on the nose wheel steering tiller before actually turning it.
Consider using differential braking for the turns and if the aircraft gets too slow, advance power on the outboard side of the turn.