The nozzle has tiny air holes that can get plugged if you don't clean everytime you finish painting, and even between colors and coats. 99% of the time, that's where I run into problems.
Backflushing, by placing my finger over the nozzle and forcing the cleaning solution to bubble up in the cup, with a water/dishwashing liquid solution (two drops in a quart spray bottle of tap water) during the cleaning process helps. Be sure and wipe down the needle each time you do a backflush, and run some clean water through the brush after each cleaning.
If I am having problems while I'm painting, I backflush gently with the actual paint, to see if it's just a small clog.
Otherwise, I put the paint back into the paint bottle, and use Windex first, and then the water/soap solution, and backflush the heck out of the brush, until it sprays well. Usually it takes once with the Windex, and then several times with the solution.
I know Iwata says don't use Windex, or ammonia, to clean your brushes because it damages the chrome, but I've found that I can use it and it helps, if I'm careful to use the water/soap solution and clean water afterward, and wipe any excess off the brush with a rag.
Lastly, if I can't get the brush to spray after that, I go to Defcon 5.
I disassemble the brush completely, soak everything except the trigger mechanism overnight in acetone, reassemble and backflush with clean acetone, again and again, until I'm tired of doing it.
Then I run a couple of bowls full of the water/soap solution through the brush, back flushing with it, too.
The Iwata brushes are thoroughbreds. All they need is good cleaning and they perform.
Once you develop a cleaning routine, you'll be able to do it in less than a minute, so it will become an automatic part of your painting routine.
Having a tupperware full of water next to your painting area, so you can backflush with water after each cleaning, makes the whole process even easier.
Good luck