It is indeed possible that in this one cell, there is one faulty MS. You just have to wait that this customer complains about impossibility to finish a transfer (or hanging transfer).
You can try to increase the detection threshold (how many times the window is stalled…)
if the problem is in DL,
if the problem is in UL.
(there are 2 different indicators : (UL) GPRS_UL_TBF_drop_N_stagnatingWindow and (DL) GPRS_DL_TBF_drop_N_stagnatingWindow )