the problem is that a flat chest press is a compound movement. You smaller “push” muscle are not as strong as your pectorals. So they fatigue more quickly which means less of a workout for the pecs. A way to correct this is to widen your grip on the Flat bench press to engage the pectoral muscle to a greater degree. You can also add fly movements and other variants of teh bench press to your workout.
another possibility is to pre fatigue your triceps and front/middle delts prior to performing a flat bench press. you could also do DB flat bench press.
there are numerous answer to your question so try a bunch of different things and find out what works for you.
bodybuilding is something that for the first few years is largely trial and error.
Jimmy