What's wrong with my painting technique?
Latex paints have a known attribute that allows certain things to persistently come through the paint even though you try applying multiple layers. 

I once had a problem of a wall that had a poster glued to it by a previous owner. I removed the poster and apparently the glue residue, however small it was, kept changing the paint I applied over that area. 

Another time I was trying to paint a door that had been finished with an oil based product. I had worked diligently to strip and sand off the old finish before painting. Unfortunately the old finish had penetrated the wood to varying degrees in different spots. The paint I applied to the door was not uniform even after four or five coats - hard to remember exactly how many as it was 25 years ago.

The fix for both of these problems is to apply a sealer / primer product  like Kilz before applying the final paint. One needs to make sure the surface is properly prepared before applying the sealer / primer. Light sanding can help and washing with a strong solution of hot water and TSP can help.