Well there could be a number of reasons.if not due to bad planning.
First check if the link balance is degraded for all the TRXs / All the TRXs belonging to a zone.
if all the TRXs then the issue might be with the antenna system / associated feeder cables check for VSWR and any sharp turns or connectors may be loose / feeder faulty or in rare cases antenna faulty.
Else if all the TREs of a zone then check the VSWR / combiner / antenna if seperate or associated cables
else if one / two indipendent TREs
check VSWR issue may be
1. Faulty TRE / associated cables.
2. associated cables.
3. TRE slot.
4. Combiner port.