I had the same problem when I started. One of my buddies helped me cure it. He explained that I might not be expelling all of the air in my lungs each time I exhaled so just a little bit of air remained on each breath. After a few breaths doing this the volume of air in my lungs built up and I became positivily bouyant and started floating up. I worked on making sure I cleared my lungs on the exhale and the problem was solved.
It could be that while you are moving you are exhaling enough due to exertion but when you stop you don’t exhale quite as much, causing the problem.