I've run into this issue in the most recent two builds of Dwarf Fortress, though it may be in earlier ones as well.
For some reason, I cannot place doors on the top row of 2 wide hallways. Or in many other places where they should be placeable.
In most cases the determining factor seems to be whether or not a door can be placed one square below it, but this isn't always the case.
For instance
The gate to my fortress, I would like to place a door two plots below my current door, but, for some reason, I can't.
Failing that, I would like to be able to place a door two plots below and one plot in from my current door. But I can't do that either. Please explain this, or remedy it.