The delt has three heads doing three different jobs. Training one is not enough, and it is nearly always the rear that is missing.
Shoulders — Chest, Triceps
PrimarySecondaryNot worked
What it is
The front delt flexes the shoulder — it already works on every press you do. The side delt abducts: it is what gives width, and only lateral raises really load it. The rear delt extends and externally rotates; it works on horizontal pulls, never quite enough.
How to load it
The front head already gets plenty of indirect volume from chest work — do not add more. Put direct work into the side and rear heads, eight to fifteen sets each per week, in longer sets. The shoulder tolerates heavy loading poorly on isolation work.
The recurring mistake
Taking lateral raises above horizontal by shrugging. Past horizontal the traps take over — you think you are loading the delt, you are loading something else.