It could be all manner of things, your best bet it to start at the beginning and work your way through the process.
First off check the paint is ok, you can do this by comparing the colour with another from MRP if you have it? If so try spraying that colour is the result the same? And/or try some different paint form say Tamiya, Mig, Mr Hobby etc, if any of these spray through the brush ok, then it's not really going to be the brush that's at fault or you..
Second and if none of the above works, it could be the airbrush/s, I use H&S and have no difficultly in spraying any paint through them. That said, some paint requires a bit more fiddling with, i.e. air pressure, nozzle size, trigger operation/opening etc. I also find water based acrylics (Mig, AK etc) harder to use and prone to tip dry, others like Tamiya, Mr Hobby, Zero paints, MRP etc are very easy, to use, clean and you get very consistent results.
MRP paints have two types, Aqua water based acrylic and acrylic pigmented Lacquer Paint, these require different thinners and cleaners, which have you got? If you use the wrong one it will give you all manner of issues. Your picture is showing what looks to be car bonnet, if you're using MRP car
These you've got the latter.
MRP lacquer paint normally doesn't need thinning, but if you're thinning or cleaning the brush with the wrong cleaner is could be, reacting with the paint.
Your brushes, both are quality tools I've not used a apex (basically a Badger) but they work very well. Strip them, clean using the correct cleaning for the paint (see above) and check them for damage etc. Set the pressure to 15psi, using a test piece, card paper etc, try spraying, nice light coats and build up. They're hundreds of "how to" vids on Utube.
Lets us know how you get on.