Caffettiera (Moka Pot) is not making coffee properly
You may have a broken or loose gasket. Separate the two main parts of the caffettiera and check underneath the top part. There should be a large rubber gasket. Over time this gasket gets harder and develops cracks: these cracks let steam and cofee out and prevent the necessary working pressure to build up. If this is your problem, you will notice that the caffettiera hisses and sputters but no coffee comes up.

You should describe what kind of noises your caffettiera is making. On an other tack, are you sure the fire is big/hot enough? If the caffettiera is really large (like, a 16 cups one) and the burner is too small it may be that the pot never gets hot enough to boil.

Lastly, it may be that the caffettiera is so clogged up that water/coffee cannot go through. This would be unusual, and I assume that you have already checked that.