Before I started to decided to go to law school, I was a pre-med major. And before that I used to wrestle. From that I learned that your body absolutely will not let certain muscle groups develop disproportionately to certain others. Your body wont let itself get too far imbalanced. The ratio between muscle strength/size is different for everyone (from what I've read its usually, with exceptions, between 10% and 15% between opposing muscles). The body can be tricked into letting it happen, but do you really want to look unbalanced? lol.
This is why often times, when you break an arm or something, the trainer or doctor will tell you to work out the opposite limb that isn't immobilized - it slows the atrophy on the injured limb.
This is a big part of why the squat comment above is true. Sometimes, to increase one muscle group, you have to seriously develop another group. The body wont let you get a big upper body, without the lower body strength to support it.
If its your chest that you need to develop, the problem is likely with your back or legs. Try increasing back strength. Don't ignore your chest, because you don't want to lose the gains you already have. So just do light lifting for your chest to prevent cannibalization. Instead focus hugely something like rowing, or squats as the previous posting mentioned.
As a side note - frequently when people seem to have plateau'd this is why.
I didn't see any pics of your back or legs on your profile. This leads me to believe you probably aren't focusing on them. I would definitely recommend that you put up some pics of those areas, perhaps one of the true lifters/trainers on the site can take a look and see if you're out of balance.
And, as the above comment mentioned - don't do such huge jumps in weight - the higher you start to lift the lower the increments you need. Thats why a good gym will have 2.5lb increments for ya.
There is an added benefit to this that people tend to overlook - keeping those ratio's tight will also help prevent injury - opposing muscles often stabilize movements even when they aren't the ones doing the lifting.
And as far as back problems from too many sit-ups, this is often caused by over development of the abs, and under development of the lower back.
Anyhow - that was probably way more than you needed
The site below has some decent ratio information.