One problem I have found with all the brushes I used.
If you paint than put them down for a bit while doing something else there can be a thickening of the paint in the cup, especially if under a hot light.
Here is what I do now, bear with me as the example might be strange.
I build a lot of large scale figure kits and we need to paint many parts the same color(flesh), etc.
I mask all the parts, mount them on sticks for spraying and insert those into my painting turntable. In short I do ALL the parts in the SAME color in one go.
Next step is to take my pre-mixed color thin it and stir it with my paint stirrer, add the correct amount into the A/B and spray several test-patterns on a largish paper-palette, adjust mixture/settings if needed.
Now spray all the parts in one go while turning the painting table, even got a table that will hold car-bodies. Once the parts are painted it is time to clean the brush, parts remain either in the spray-booth or get moved with the stand(got 2) into the drying booth.
Alternatively you can hold each part paint it, return it to the stand and to the next one.
If you run out of paint, clean the A/B and thin a new batch from the pre-mixed color.
HTH.