Hisham,
I've been experiencing the same for a while...but this is what did it for me:
First the realisation that just about every color has its own "special mix" of thinner and air pressure. My paint is always acrylic: Vallejo & Ammo mostly, with some others in there like Hataka. Experiment and get to know your paints;-)
The second realisation was that the receiving parts have to be very clean. Whenever I'm about to put paint on I wash it first...just water and soapy stuff and more water. Plastic as well resin! The important thing that made a big difference is to wear gloves...the ones without the powder (the kind that doctors use when...). Your naked hand will leave a residue on plastic that makes the paint want to go somewhere else...
I usually use a primer, but I notice that when you spray on clean plastic you really don't need it anymore. This has the big advantage that a "thick" primer can hide that subtle scribed detail you worked so hard on.
Both, with and without primer, work just fine for me.
As for the Mr primer working for you, well have a think just how aggressive that stuff has to be to do what it does...
H.