If you can do pull ups with out any problems then it sounds like a technique problem

Few points to consider for the muscle ups...

1) Are you training with a false grip?

2) Are you pulling towards the chest instead of until above the chin?

3) Are you training horizontal dips?

If no then do that, If yes it's probably that you don't know how to transition from the pull up to the dip. Try this: As the bar reaches your chest, go in close and arch you upper body towards the bar so that your shoulders are above the bar. IT should make it easier to push yourself up now that the weight is transfer towards your arms.

You need to realize that not everyone is the same. We are not all the same age, we are not the same height & weight, and we certainly don't start of with the same amount of strength. Some people may seem slower than others while some are faster with it when it comes to strength training, so don't worry about it, so just keep training hard and you will get it.

Push ups don't work your full body weight. If you want to train for a muscle up and if you have problem pressing yourself up; Go do horizontal dips. (I've said it in my previous post somewhere above)