It sounds like one of the cell tabs where its soldered is less than it needs to be and is a flaky connection.

You can try balance charging and see if the connection will stay connected long enough to charge it. But then it also has to stay connected for you to run it as well. Worth a try, can't see it hurting anything. This lipo is not going to be reliable though. It may take just a little shock to lose connection with that cell again.

If that cell loses connection, charger will stop so it stays safe.