That sounds like an orphan, where someone else has found a block at almost exactly the same time as you did. Only one miner can win.
If you do 'listtransactions' in the debug console you should be able to see the orphaned blocks.
Yep, spot on. Thanks for giving the command, am seeing an 80% orphan rate.
Am not used to seeing the rate of orphans this high, it's usually the other way around. Suspect it may be down to general network latency combined with my low hashrate.
EDIT: I am based in Australia so this is no surprise TBH