If you're doing a ton of pressing movements throughout the week, it's probably the former, so give your delts some time to rest and grow. If you've been coasting through infrequent shoulder training, it's probably the latter. To kick-start them, try doing drop sets on one exercise per shoulder workout for a while. For example, you might try doing shoulder presses with dumbbells that allow you to complete six reps using good form, then immediately set those down and complete another six reps using a lighter pair.
Before beginning your working sets, make sure to warm up your shoulder joints thoroughly. You can do this any number of ways, but I prefer doing reps of a compound movement, like the bench press, using just the bar. I also like doing internal and external shoulder rotations using 3-5-pound dumbbells. Basically, you want to increase the blood flow into the shoulder joints before training them hard.
Shoulders don't require real heavy resistance to grow. I can do seated machine presses with the pin at the bottom of the stack, but I typically use light weights. A good rule of thumb on shoulder presses is that if you feel the movement in other parts of your body, you're going too heavy.
Take your shoulder exercises through a full range of motion whenever possible. The ball-and-socket construction of the shoulder joint allows for great mobility, and taking your exercises through those ranges of motion beforehand will lessen the chance of injury on the playing field or fitness stage.
Don't squeeze your shoulder blades together during rear-delt movements such as bent-over laterals. When you do, your rhomboids and traps, which perform scapular retraction, take much of the load away from your shoulders.
Don't assume you're using proper technique just because you haven't suffered an injury yet, especially when it comes to shoulder training. I've heard people say, "I reached out to grab something off a store shelf and I injured my shoulder." No, you didn't; you probably accumulated years of progressive damage from bad form, and grabbing that soup can was the straw that broke the camel's back.
