Humbrol paints are now made in China and some say they are not what they used to be. I think there is some truth in that. There does not seem to be the same amount of pigment as before there have been complaints about slow drying times. It is essential to thin them for airbrushing. The drying problems seem to be related to the paint being too thick or too thick a coat being applied, especially the gloss or semi-gloss colours. Even then the paint can dry slightly sticky. I have experienced this myself. I now add terebene paint driers every time. Their own thinners, white sprit or lacquer thinners are all suitable. I use their own brand which seem to be formulated for airbrushing, it's not just white sprit. Stir the paint until your arm hurts, then shake the tin and stir it again. Even if yoyu think you've stirred it enough do it again, especially mattcoat.