It might be to do with the quality of them, (watch it, you will blow me out by telling me that they are a good brand). I have played with Chinese brands in the past and they tend to do this.
One thing you could try is to put a heavy load on them, don't short them, and run them completely down. Then try charging them again, on a slow charge if you can. Repeat a few times.
I have some Saft (expensive industrial make) NiMH any they are now about 5 years old, had lots of abuse, not touched for a year at one point, no problems.
When ever you do charge your batteries whether it be Li-Ion or NiMH, if you can afford the time, charge them always on the slowest charge (ideally Capacity / 10 for NiMH) as this really prolongs life. Fast charging reduces life times.
Mr Mike www.mineexplorer.org.uk