The door to the ATST at the end of level 19 very rarely seems to open. It's only opened once for me (via backtracking a bit through the tunnels and returning, but I can't figure out what I did differently that time - backtracking doesn't seem to reliably fix it).
The door is stuck due to AT-ST trying to walk through it. I managed to open it by shooting rockets into the little opening until the AT-ST died and the door opened.
The door to the ATST at the end of level 19 very rarely seems to open. It's only opened once for me (via backtracking a bit through the tunnels and returning, but I can't figure out what I did differently that time - backtracking doesn't seem to reliably fix it).