Working your medial deltoids is quite different than working the other parts of your shoulder.

The muscle fibbers are shaped different and don’t necessarily need heavy weights though they can handle heavy weights.

I would suggest lighter weights high reps and strict form.

Execution:

Stand holding a pair of dumbbells to your sides.

Slightly bend your arms, then as you are raising them begin to turn your hands down like you are pouring a jug of water.

Your arms should be parallel to the floor.

Bring the weight down slowly resisting all the way down.