I use nothing but Vallejo Primer, and believe me it isn't soft once it is cured properly. How soon after you put the primer on did you put the color coat...and what was the color coat?

Vallejo primer needs a clean surface, free of hand oils and mold release oils, spray it on, let it cure (24-48hrs) and it is like a rock once cured. One of the few paints that will give you fits trying to strip it off a model because it adheres so well.