When you can do things in exercises, but not in an actual picture, it's quite often because you start overthinking things, as you are worried about ruining the picture. Then you can become tense and lose your touch. So just go for it and don't worry. You may not always get a perfect picture, but you will get elements you can be proud of, which is awesome as it shows you are starting to nail techniques, which is the basis for every future painting you will do, and probably areas that didn't work so well. This is all good, as you will then teach yourself how to do it differently next time, or learn to correct things. This is a really important part of learning to airbrush. At some point everyone messes up, and knowing how to compensate, or correct or adapt is vital, otherwise a lot of hours of work can be wasted by a tiny mistake. Never be afraid to ruin anything, just go for it, and if it doesn't come out exactly as you hoped, then it's a great opportunity to experiment with how to deal with changing/fixing things. (Mind you I stil enjoy a little swear before I have to fix anything LOL) So just practice as much as you can, you're on the right track!!!!