As Husky's Dragon says, an intro would be nice to see
and if you can put your location in your profile that is also helpful, occasionally we have found members live nearby and can offer local knowledge
now, down to your issue.
What PSI are you spraying at ?
is there a chance the regulator is set to low, (set the PSI with the trigger pushed on your airbrush) is your chair on your air-line (don't laugh, i've done it and been scratching my head for a good 10 minutes)
here is also a basic check
Clean your brush of paint, add water - I'll make a suggestion here, ALWAYS do a test spray with water before starting a paint session, it will save you some paint and show any issues.
Can you spray water ?
No - you likely have a blockage, it doesn't take much, the paint path is very narrow and a microscopic dried piece of paint can cause major frustration.
Yes - then your paint is likely to thick or has found a lump, try upping the PSI or thinning/straining your paint.
Is the vent hole in paint cup lid free of blockages ? a vacuum will form and stop the paint from flowing if its blocked - try painting with the lid off and see if it makes a difference. .
did you remember to tighten the needle chuck with the needle all the way forward after you reassembled. Don't force the needle into the nozzle (you'll crack or flare it ) but it needs to make contact and a good seal. does the needle feel as if its 'sticking' in the nozzle if you pull the needle back and forth when you reassemble (not using the trigger) if so then the nozzle is dirty and needs a good clean (yes we know you cleaned it, we've all claimed we've cleaned a brush, only to find that after the 5th clean it actually was clean ! )
Chapstick or beeswax on the threads of the air cap/ head area can help seal those threads. you only need a tiny bit on a cocktail stick wiped into the threads, to much and you'll potentially create airflow problems.