Are you turning it over to get some air into it? I know this can be difficult with some of the shop bought compost bins because they are too tall and narrow to get your fork into, but you really need some air in there.
If the compost bin has a lid then it could be to dry, so try to make sure that is always damp.
I always smash egg shells up completely before putting them in the compost, otherwise they will take an awfully long time to break down.
If I am putting vegetables or anything large into the compost bin then I always cut them into small pieces before adding them, it takes a bit of time but it really speads up the composting process.
I also add all our coffee grounds and tea bags to the compost bin, it helps to give your compost that compost color quickly and helps to keep the pile damp.
Finally every week I get a large newspaper shred it and add it to the pile, you could use shredded paper from an office if you have access to it. I have no scientific reason to do this, but I like to think that the paper will absorb the nutrients from breaking down organic matter, and it will also keep some air in the pile.
A few years ago I was in the same situation as you and the garden center sold me some pellets (compost activator?) which was supposed to get the pile to break down, they did nothing, and I have found that all the things I have said above got my compost to break down quickly.
Also if you look at the bottom of your pile you might find some decent compost, and it just the organic matter that has been added recently and visible at the top that is yet to break down.