I actually used to have this issue as well when I first started lifting. Just my own experience – I found that I had really poor control over my pecs so I started practicing contracting my pecs randomly throughout the day – basically doing the “titty dance” to work on my ability to control the contraction. Once I learned how to contract on que, I could apply that during my lift. I also found that doing resistance band flyes helped some as well with learning how to contract.