Badger 250-b airbrush

Have you tried adjusting the height of the nozzle?

On the 250 you adjust paint flow by turning the nozzle to raise/lower it. If it's too low and the tip is out of the airflow, it won't pick up paint.

Conversely, it can be raised too far and again, will not pick up paint.

I adjusted it a few times... YES! It worked, I tried that but I guess it wasn't high enough... Many thanks!

Good to hear it's working for you.

There is a tiny paint passage within the body of the brush that can clog with paint.  The plastic tube may be paint free, but there might be a clog in that passage within the brush. If old paint has dried in there, it may take a strong solvent to dissolve it.  While acrylic paint can be thinned with water, the dried paint is quite waterproof, and in fact it withstands many solvents pretty well. If you used acrylic you may need to resort to lacquer thinner.