LIPO batteries cannot be shelved for a long time without being recharged to the storage level charge (70% I believe). You should never store batteries fully charged. if you have a voltage meter, you could check cell levels. If below 3V, then the battery cannot be recharged. Try a balance charge instead of a normal charge, and use a very low charge rate (say .1A) if your charger can handle it.

I have seen other tricks in other forums, but don't remember where...