How to clean brush after polyurethane?
I have had trouble cleaning them with everything and had the same problem,  until I found a jar large enough to put the brush in filled to the bristle level with thinner keeping it tightly sealed. (I do clean them prior to putting in the jar).
When I need them take out shake dry and they work like new brushes for months without being used or when I have a project that gets a new coat every day for a week. (I find the higher quality brushes provide the best coverage so I hate to toss them with just 1 use)