The UE never responded to a request the network sent it (UE not responding) -- the failure is on the radio/UE side, not this GTP-C exchange itself.
Where you see it
In the Cause IE of a GTPv2-C response (Create Session, Modify Bearer, Delete Session…) on S11, S5 or S8.
What to check
Check the radio-side (RRC/NAS) logs for why the UE didn't respond -- coverage, device issue, or a prior radio failure.
Check which node sent the response (MME, SGW or PGW): the cause belongs to that node's decision.
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