i have the same problem. after 2-3 years of being "that guy" who was doing chest/arms every other work out my "clavicle-pec" is underdeveloped compared to the lower pec and front delt.
i have the indentation too but a couple things that have helped me feel it working more and actually got it sore were:
low/normal incline presses where you arch the weight back towards the head kinda
explanation from stu is better
"I mean arcing slightly towards your head during the concentric portion of the ROM (with a bar, or with dumbells). Many people bring the weight (bar) down to their upper chest, and then press straight upward. However, if you bring the bar lower on your pecs at the bottom, you will find that you get a much fuller stretch throughout your pec region, especially in the upper portion (Yates mentioned this somewhere). Now, when you press upward, in an arcing path in order to get the bar above your head, you will feel a better contraction from the muscles just below your collar bone."
low-to-high cable flyes from that "how to build a bodybuilder chest" article
pausing at the bottom of OHP's then exploding up