If the tomb is clear go out and get in again, maybe appears a 'missed' droid or this time you can open the door. 'Once you've reached the top of the ramps, if you try to open the door but there are still droids in the corridor behind you:

I don't remember find this problem in any of my playthroughs but I've found a walkthrough telling that if there is any droid remaining in the tomb the door stays sealed: