As Dave said, this can happen with too much pretreatment, but it may or may not be the problem. There are several things that could be happening.
1) Too much pretreat: If this is the case, look very close at the white specs. If you have a loop or a magnifying glass, look as close as you can. If it's too much pretreat, there will be little holes where the white specs are. If there aren't holes, chances are its not too much pretreat.
2) Not enough pretreat: This can happen if there isn't enough pretreat and binders to matte the fibers down. If this is the problem, typically you will see the white not being white enough, even at two passes. If it looks patchy or not a solid white, then put more pretreat on.
3) The shirt itself: Some shirts are just not good to use with DTG. No matter how much pretreat you put down, the fibers don't like to lay down. If you have all variables in line with the correct procedures and you still have these issues, change the shirt style.
4) Not enough pressure in drying pretreat: After you pretreat your garment, it's better to put a lot of pressure on the shirt in the drying process. The more pressure, the better the white. The problem is with more pressure, it can leave a more defined marking, so you will want to do very light pressure for about 10 seconds to lift the moisture, then heavy pressure until it dries.
5) Wrong kind of paper: Some people use paper when drying pretreat. Some types of paper are better than others. If you are using paper, lifting it off the shirt after dried can pull the fibers back up. We use teflon. I don't like the teflon sheets, they are harder to handle. If your heat press can have a teflon cover, I'd put one on it. The teflon works great for drying pretreat but when curing shirts with white ink, use a sheet of parchment.
In all the time I've printed, I've never used a brush or roller to matte fibers down. People do it successfully, but I've always looked at it as a waste of time when I'm trying to do production. If it works for you to use a brush or roller, go for it. Using techniques as described above, I can get a great print with no fibers sticking up.
Good Luck!