If you train chest an extra time, you could be over training the muscle. Maybe chest needs more rest than other body parts. Also, you can try pre-fatiguing the muscle. Like pet deck for 5 sets of a hard 10-12 to failure. Do this before you start doing your compound lifts. Also you could be using more of another body part to assist and not even know it. Have you found that your shoulders have responded well? This is super common. In that case, bring your elbows in closer not flaring super wide and focus on the contraction of each set for pecs