Check the connectors.

I had this happen after the small pins in the TX were bent due to frequent removal for LiPo charging.

The problem was not with the LiPo itself but with the momentary disconnects from power.

I ended up going back to NiMH on one TX just to avoid removing the LiPo pack.

On a TX I cared less about I rigged the balance connector to be accessible with the cover in place, so as not to move the pack.

That way I can charge the LiPo with the pack in place... though this is not the best way of doing things...