My armour suit ran out of juice. I've got 10 fusion cores sitting in my inventory. It won't automatically replace spent fusion cores. I cannot manually put a fusion core into the suit. I can take out armour pieces, and put them in. I can repair armour and even make modifications, but I just can't use it because it won't let me power it!

I know this might seem stupid, but are you sure you're using cores, and not accidentally trying to use fusion cells? People have gotten these confused.

Would have to agree with the above poster. My dad was saying the same thing to me that he had 37 fusion cores but couldn't seem to power his suit. Turned out to be cells for the energy weapons.