Having more than 7 units in a hex is supposed to create slowdown but the units should not stop. We've not gotten documentation of any major problems with the movement through stacks but it is possible there are some glitches there. What kinds of units are entrenched and what are you trying to move through?
I was playing Central America campaign as Costa Rica and the terrain was alot of close combat hexes and mountainous. I had a city I entrenched with 7 units- 3 light infantry, 3 artillery and 1 AA. I also assigned them all to group I and had them entrench a small town near my border with Nicaragua to the north.
when war was declared I left those units there and selected a groupof infantry and artillery from my capital and had them all move to a position past the entrenched town.
I watched as the game generated green arrows for them which followed the road through that entrenched town and to the position I selected and I watched the units travel along the road and once they reached the entrenched town they froze. The green arrows remained, indicating they still had their orders to move to the position beyond the town, but they just refuse to move. I selected a few of the units individually and repeated the move and some did move ahead others moved a very short distance close to the entrenched town and froze again.
this was not the only instance during this game. it happened on other towns that i had 7 units entrench a town along a road and tried to send units through and beyond them. I cannot reproduce it 100% though with concrete step-by-step instructions, or at least I have not yet figured out how.