Tamiya dries tacky

You may have had some dirt or chemical on the surface you painted before you painted it.  The most likely culprit is Impatiance, Dont get me wrong we have all been there.  How long did you wait before adding the light coats? 6 hours is way to soon to judge weather your paint will bond. I at the least wait 24 hours before I touch anything I have painted.   If i use acrylics I expand that to about 4 or 5 Days 

Besides what has been mentioned I can add that X-20A has a little retarder in it. The paint could remain a little softer than full cure but you also won't get tip dry when spraying it. It's hard for me to judge because I tend to use a little retarder anyway in my spray mixes but I also don't fully just air dry. I use a dehydrator for the initial kick start so to speak. And too it depends what you mean by tacky. If you mean it just doesn't feel fully dry but you can handle it it will probably be fine tomorrow or so. If you mean literally soft and sticky and you can't handle it, well I wouldn't expect that after 6 hours and something might have hickupped.

How long did you wait between applying the primer and airbrushing on the paint?  If you did not give adequate time for the primer to dry and cure before adding the paint, the problem is likely there.