Alan
Ae have four poles going as well
NWH
I know they fail for all sorts of reasons, today i had one fall to bits, that was a first.
They get damage whan you drop them from height, some stop retracing , some get clogged with grit etc.
We used to push then in manually to fill our bucket, and if you pushed them in too far they would stick.
The most trouble we have is with them warping when dropped from balconies etc, now i usually take them down the balcony with me instead of just dropping them.