Most STP problems reduce to missing BPDUs or bad links.

Symptom	Likely cause / tool
Blocked port unexpectedly forwarding	missing BPDUs → Loop Guard should have held it
Up/up but one-way traffic	unidirectional link → UDLD
Duplex mismatch errors	fix duplex settings
MAC table flapping	suspect a loop

show spanning-tree
show spanning-tree inconsistentports
debug spanning-tree events

Habit: “no BPDU = suspected loop”. Don’t only inspect forwarding ports.