faded modelled look on spot colours?

[gabagoo]

What can cause that fade out or modelled look when printing spot colours? Is it to much heat? or ink starvation? If it is ink starvation how does one remedy this situation. I dont get it all the time Ijust dont know wen it will happen or what causes it. I see it a lot on banners but yesterday I caught it doing it on regular vinyl.

 

[Letterbox Mike]

Sounds like surface contamination to me. Can you post a picture of it?