We think it died because it had problems at birth but we think the mother relocated it but we can't find out where

Indoors or outdoors?
The sad fact is, if outdoors you may not. You can attempt to be sneaky and watch mom carefully to see where she goes. However, if you follow her around she may not give you any clues.
If there was something wrong with the kitten, then mom may have abandoned her or worse.
That is mother natures way.
Very young, tiny animals will go downhill really quickly without nourishment.
So try looking around and listening carefully, but try not to get your hopes up too high.
I'm not trying to be negative, but finding her may prove difficult at the very least, especially if outdoors.

Its indoors and she hasn't shown any sign of it being gone

I'm sorry, but if the kitten was very, very young and had problems it is highly likely that the mother may have eaten it. They do so because they know it won't survive and their instincts tell them to eliminate it so nothing will come and harm the rest of her babies. As LadySam said... it's Mother Nature's way.

If Mama doesn't show any signs of looking for her kitten, or disappearing for periods then I would strongly suspect that's what has happened. If she does leave the other kittens for periods you must watch her because she may have the other kitten hidden and be travelling between two places. A few common spots you should be aware of are inside the back, bottom or sides of a couch or chair, the bottom of your mattress, a pile of laundry, in a cupboard, etc. However, it really sounds like the kitten could be gone.

Hugs, Didi