I use Humbrol Matt Black spray for undercoating and their Matt varnish for, well, varnishing.
Can't say I've noticed any problems recently, and I find their cans a lot more reliable than they used to be. I haven't actually had one clog up on me in several years. And yes, I know I've just jinxed the heck out of that and I suspect the next time I even look at a can it will explode

I used to use their black from a pot to touch up where the spray had missed, but something changed with them. Where they used to be matt, the last couple I tried dried very satin, verging on gloss and didn't cover very well. Any other Humbrol pots I've tried haven't been up to much - although no where near as bad as the problem you describe.
The pot of paint you bought could have been a bad batch, so it may be worth another test. Having had two pots of black equally rubbish, I've given up on their pots. If I now start to get problems with their sprays, I'm blaming you
