One downside of wireless repeaters, though, is that they reduce throughput on the WLAN. A repeater must receive and retransmit each frame on the same RF channel, which effectively doubles the number of frames that are sent. This problem compounds when using multiple repeaters because each repeater will duplicate the number of frames sent. Thus, be sure to plan the use of repeaters sparingly.
Just a thought .. did you survey the location for the repeater before it was setup.. If it cant hear the AP then it can't retransmit valid data..
is useing different antenners an option on the particular Repeater? most antenners don't hear to well directly above and below themselves.. ie if the repeater is directly below the AP.
Also you could be suffering hidden transmitter syndrome.. that is where your Repeater is trying to relay back to the AP, but the data is transmitted over by an other source that your repeater can't hear..

so.. 1/ you need a good Rx signal at the repeater to do any good
2/ consider the likleyhood of heavy users above blocking your repeaters' signal
3/ would it be better to use 2 AP's , one on each floor..ok will require panel antenners, and cable to the lower level

cheers..