If you have been infested by the parasite the DNA scanners won't let you through those doors. Depending on what you did before being infested this can cause you quite big problems (but you can still solve them). If you are stuck on one place too long try one of the other places for a while. Doing that can often open up solutions in other areas. Also depending on what you do with certain items at the start of a dream they open up different pathways through it (while closing others). For example which lock you decide to pick in the lab.