I've worked with many clients who've had trouble getting their pecs to fully contract (particularly the upper region), and admittedly, this is something that I used to struggle with as well. The below exercise is one that I often incorporate into their training, which has a solid success rate for helping overcome the issue. After the first set, a client's face often lights up when they realize that they are experiencing what a full chest pump feels like for the first time.
The goal here is to activate every fiber in your chest. Grip the outsides of a bench with both hands and push them in, creating a lot of tension/pressure. Keep applying pressure as you lower your chest towards the bench. Pause for a second at the bottom, and then lift back up by trying to bring your hands together, as opposed to just pressing up like you would with a standard push-up. When executed properly, these can be tough, as a set of 8-12 could take you close to failure. Add them to the beginning of your session to help establish a strong mind-muscle connection right out of the gate, which I find carries over to the chest work that follows.