Lipos do die after a while. Some sooner then others. Best thing to do is top off your batteries right before you fly and do a discharge cycle (if you have a lipo charger with that feature) in my opinion after 2-4 cycles on that battery. Yes they do lose charge if you haven't used them in some time, that's why it is best to top it off before you fly.
Another thing I have found with my Phantom V1 is to not plug in anything from the main battery till the drone is all warmed up and ready to fly. Given I only fly with a Walkera G-3D gimbal, it still saves me some battery life. No sense having your gimbal keeping stable while your not flying. Same with your FPV