Wither skeletons can't move when a block is above them in the overworld/end. This is not the case in the nether. The height of the block above the wither skeleton has no bearings. As long as the block is above the wither skeleton, the wither skeleton can't move. Blocks like glass on the other hand do not effect the wither skeletons this way. Depending on the situation, wither skeletons can move with a block above them.