Firstly, if you haven't lost weight, I would say check what you are eating. Exercise and food intake go hand in hand. I exercise a lot and I haven't lost weight recently, and that's because I am eating exactly what my body needs, I am not creating a defecit. It's really easy to overeat when you exercise, or it is for me anyway, because I physicially feel hungry. When I was 262lbs I never felt hungry!!!
As for crunches, you need to exercise the opposite muscles as well. Your abs wrap right around your back. Do you have stability balls in your gym? If you do, get a trainer to show you some exercises, there are loads that will improve your overall core strength, which means tight and toned abs. Back exercises like supermans, where you get on all fours and extend opposite arm and opposite leg and hold it, will help. I do all my abs work on the stability ball.
Finally, your pushups will get better and better as you get stronger, just keep at it!!!
Hope that helps.