+1 to this above
I am currently using whats left of my NXT then i'll be going for something else. To be honest it sounds like you might be putting too much on - it could be as simple as that. When i first tried NXT i had a similar experience to yourself, these days i'll give the car a few coats of Autoglym Extra Gloss Protection then some NXT which can then be applied in as many 'thin' layers as you want.
Other factors to consider could be the outside temperature when applying the product.