I went into the Nether on a glowstone run and when I returned, I noticed my MFSU was not filling up from my reactor. The amount was just staying exactly the same. no machines were running and there was no draw on the MFSU. I checked to make sure the wiring was intact, and the uranium cells still had plenty of life in them.
Taking out the cells and replacing them did not fix the issue. I exited the game and restarted and it seems to work OK now. I'm not sure why, but it would seem that travelling to the Nether and back does something to it.