I've had EXACTLY the same problem after using Tamiya acrylics Matt. I eventually changed the whole spray head, as I have a spare Badger 'Fine' tip from a worn-out airbrush. Meanwhile, I left the 'Medium' tip soaking in enamel thinners for 48 hours, then dried it, and gave the needle a VERY LIGHT rub with fine wet and dry, and cleaned out the tip nozzle with fuse wire and a paint brush, then washed the lot in enamel thinners. It seems O.K. now, and I think it was just 'dust' from dried paint interfering with the flow of paint/air.
It doesn't take much to make things b****y awkward!
I can just see in your photo that the needle seems a little coated, but it might be the lighting; all I can suggest is try what I did, then blow through a LOT of enamel thinners. If that starts to work, but doesn't fully cure it, try soaking the whole assembly, stripped down, in some cigarette lighter fluid (petrol type), the kind you'd use on a Zippo lighter. A bit radical, but that stuff cleans anything! Make sure you let it air dry, then blow a bit of thinners through it, before trying to use it again. Obviously, no naked lights!!!
Terry.