Bridge Ops in M44 and B45

[FLG]

"and in M44 not one unit has managed to take down a bridge and I am on turn 20"

I think you must be unlucky. I am playing the Russians in M44 campaign and have had plenty of engineers put up and take down bridges and we are on about turn 28. It can take a while to take down a bridge but I have seen it happen in one or two turns. On average I would say it takes 5 or 6 turns.