I'm no expert, but if your biceps are bigger than your triceps, I'd say you're definitely spending way to much time on your biceps (just like every. single. guy in my gym). What has always worked for me is doing more close grip bench presses and BARBELL military presses, I stay away from the machines.
This happened to me one time also:
I was so wrapped up in kickin up the weight on my bench that my form would suffer. I would jack the weight up week after week, and I was using less and less chest and more and more shoulder's and triceps. I was lifting almost every set to failure with agonizingly slow reps, trying to get more than the last workout and finally it was as if my bench press just "broke" in a way, lost all kinds of kilos, it was very demotivating and I felt stupid and had wasted a lot of time, all for ego basically.
Like I said, no expert here, but this is how I fixed it: I dropped the weight to where I could do good form, bringing the bar to my chest, stopping 1 second and PUSHING explosively. This way I made sure the form was correct every time before moving on. The bar should move fluidly, not slow and agonizing, before you advance. I also did 3-6 reps ie:8x3, 6x4, 5x5, 4x6, and when I could do 4x6 with PERFECT form, pausing for that all important second on the chest, I knew that the next time I would put another 2 1/2 kilos on and do 8x3. It might seem to slow to advance, it certainly did for me at the time, but it pays off in the end. This style of training coupled with more close-grip bench and BARBELL military press work did the trick for me.
I've regained, plus much more now, and I have good form. I know I could bench a lot more with the form of all my fellow gym-mates, but I'm not there to out-do anybody (even though I do with good form anyway). The bench I think is a very slow exercise to make solid gains on, so just take it slow, but steady and you'll have more success in the long run.