Dry Transfers are waxy, which is what makes them stick. Make sure they are warm before applying. They burnish better on flat sheen surfaces, it gives the waxy material something to grab on to and hold. CAUTION! Some clear coats will cause them the breakdown and run especially if applied with a brush. Airbrush any clearcoat lightly, multiple layers if necessary.
Make sure the surface they are being applied to is oil free. As I said warm Dry Transfers stick better, body temperature is sufficient. I will set mine on top of the fluorescent lamp above my bench which has a radiant temp of just over 100F. Thirty seconds or so softens up the Dry Transfer, making the back sticky and ready to be burnished on the model.
I like Dry Transfers because they are easier in most cases than decals to apply. They weather more realistically too. They are also great for masking too, but that is for another post.