Some people don't get good pec work from flat bench, and especially push-ups because they are/should be pretty easy for most lifters, so the simplest solution could be to choose more effective exercises. DB benching on a low incline and/or slight decline, and dips, may be better options.
D Roy raises, 2x6-10, as an activation exercise before any other chest exercises. It's activation, not pre-exhaust, so you don't need to kill yourself on them. Each rep should get a good contraction so your brain says "Oh, that's a pec contraction!"
The Svend press, sometimes called a plate press, is another activation exercise some people use instead:
Also check your form, load, and rep speed on every chest exercise you do. Going too sloppy, too heavy, and/or too fast are easy ways to avoid targeting the intended bodypart.