Deltoids can be really difficult to grow for some people and others they just seem to hang off. Part of this is always going to be a genetic disposition but that doesn’t mean we can improve them regardless of this.
For example, rep range & tempo is crucial, especially the eccentric/negative phase of the movements. I often see people just banging away with no control or contraction of the muscle.
What’s the alternative? Try lowering the weight slightly and ensuring the shoulder are working eccentrics movement as well as the concentric, this will mean better engrailment and less stress on the joint and ligaments ensuring longevity in the gym. This doesn’t mean you are lift light just more controlled and with a eccentric movement a greater pump.
The main point I want to highlight is, How well rounded is your shoulder training?
I'm a huge advocate in a 3 dimensional training approach. On shoulder day this means hitting the deltoids from all angles.
The biggest mistake I see on shoulder day is a lot of emphasis on anterior (front) and lateral (side) exercises and not enough on posterior work.
What this creates is an unbalanced LOOK from an aesthetics point of view, but also a recipe for injuries due to excessive internal rotation of the shoulder joint.
What I'm saying is you MUST make sure you spend as much time training the posterior portion of your deltoids. Remember, on chest day your anterior area of the shoulders will be getting hit anyway, which is all the more reason why its KEY to train the posterior region properly.