check the avg nb of TBF per PDCH. too high ?
check the average throughput per TBF. too low ?
check the average number of PDCH busy during the busy hour. is it equal to the MAX PDCH ?
check UL or DL TBF estab failure due to radio congestion. too high ?
if answer is yes in one of the question, you can try to increase the max pdch
beware that it’ll work only if the nb of busy TCH at the GPRS BH gives enough room for more PDCH.