Ok sounds like something is certainly off. A lot of times, it's form related. With my online clients, the first thing I would do is have them shoot a quick video, showing me how they do a working set. From there, I can see fairly quickly, whats going on, and what needs to be changed. Of course, we don't have that luxury here, but I think I can still help. In order for a muscle to grow, you need to work it, right? So what I would suggest is that you start by backing the weight down and put your energy into contracting the pec muscles. Dumbbells usually make it easier to squeeze during presses. Another option would be to start with an isolation movement like pec deck to pre activate the muscle. This way the pec is more likely to wear out before the triceps

With Biceps, I would train arms on their own day. Give them some attention when they aren't tired from back training 

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