I have had a few problems like this. I have found that if you are getting brush lines and bubbles. It could be a few things. The most common being: the paint may be too watery. So you may not be seeing the brush lines from the current paint, it could be the brush marks from the previous color that dried, and the bubbles are from the paint being used.
The other thing that could be going on, maybe at the same time, is simply the brush you are using. A large flat brush is preferable to these large areas, but also one that has good moisture management. This is a trait that is hard to describe, but simply if you put water down and your brush can pick it up and then when you paint with just the water it should let it out slowly. If it is not good at this the brush will not pick up the water well, but when dipped into a cup of water it will just let all the moisture out in large globs because the brush has a hard time absorbing he water so it is mostly just water on the outside of the brush. Test this to see if it is not just the brush, and then if not the brush try mixing the paint with another paint instead of a wash. That's my best recommendation. That or go over the color with the wash after you paint it.