hi,
This kpi is per TRX, so isolate the TRX which shows poor efficiency.
if the RTCH efficiency is below 90%, then the probable cause is a TRX or a combiner issue (–> Hardware problem)
if the RTCH efficiency is above 90%, then you should investigate frequency interference, or perhaps a little difference of coverage between the BCCH TRX and the RTCH TRX (–> Radio problem)
regards
pix