Bad experience with Humbrol acrylics....

Hi gents.....

Was working on my SAAF Spammy today (For the first time in ages).. and after a nice long session of masking off the wings, I mixed up some of the Humbrol dayglo orange to airbrush.

Got my new nozzle out, loaded up a cupfull (with a bit of thinners as it looked a bit thick)... and began painting...

but GASP....

nothing happened.

No paint. I check the cup and the paint had turned into a sludge that promptly clogged my nozzle. I then cleaned up and mixed up a newer batch, but with more thinners.. and the same result. After some persuasion I managed to get some flow through the nozzle but it was very intermittent flow. Sometimes thick, sometimes nothing. 

Eventually I covered a fair bit of what I wanted to paint... and called it a day. 30 min later I checked the coat.. and it had formed a rubber seal (same as maskol would). I promptly started peeling off the stupid coat and now I'm having a rethink.

I've never had this issue with Tamiya paints. Ever.

Have I done something wrong??