If you have been spraying acrylic that could be the cause and you will not be the first to be caught out, nemesis
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The fact you've got air coming through your cup means your nozzle is definitely not blocked.
If you've got air coming through your paint cup, it's because you've got an air leak into the cup. Check your nozzle is seated correctly if it's self centering, if it's a screw in check your o-ring is in place and that the nozzle is in properly. Check your nozzle cap is on properly and again if fitted with an o-ring (I've never seen a brush that hasn't got a seal on the nozzle cap) check that's in place.
On another note, what you have described is what is used with cleaner to backflush. Backflushing is a critical tool for cleaning your airbrush, which works by putting cleaner into your cup, loosening the nozzle cap and blowing air though, this bubbles your cleaner through the nozzle tip and back through the cup.