I have realised that what I thought was a sporadic issue now seem to be a bigger one, and that is people paying for account upgrades are not being upgraded.
The PayPal part is working - payment received, receipt issued etc.
But XenForo is showing no sign of the result.
I moved to Xenforo from vBulletin in January, and there was a number of upgrades made on XenForo (so think I can rule out an upgrade issue) but they stopped shortly after that. No further upgrades are shown as active.
The last transaction in the log shows:
Information: No txn_id. No action to take.
Only log entries that involve a successful payment will be attached to a user and user upgrade.
However that was in Jan - there is no further log entries at all.