In FPSC where paths are concerned which pass through a doorway characters can have a little problem in the way you have pointed out.
A couple of things that may and only may help :
Dont have too many characters active or complex paths crossing or near each other at any one time or location.
When placing a character in your level try placing the character both by selecting and placing whilst Hitting the retun key to force the character to ground level and also if that dont work by placing without hitting the return key. The last option I would try first. Right click character and go into 3D inspect mode and see if his feet are off the ground - they may need to be so that he can move better when encountering obstacles below his feet. Its a useful tip for other scenarios anyway.
A characters weight and friction can imapair their movement - personally I set them to : 500 which I find is best in my case.
Paths and path points can cause a character to stop when they should not try adjusting the path points positions inside outside of any door - try changing the angle of the path a little or adding points so that turns are not so sharp.
Low fps will cause entities on paths to stop moving - full stop! - as their behaviours become unstable in complex levels with much sharing of AI think time going on. Characters will stop if fps is low and start moving agin if it increases significantly as the engine struggles to catch up with what should have been happening and as the burden of math is eased and released back to the game more evenly.
Characters need a lot of room to get through doorways and past other obstacles too or they can get stuck anyway as their collison is not 100% perfect and they dont have perfect object avoidance routines attached.
Just a few things there and there are more things than that which could be the cause.
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