do a voltage depression check on the cells - -
I use a Horizon meter which allows .5 or 1 or 2 amps load
On 2 amps a NEW Eneloop 2000ma pack (which some guys are buying as rx packs), shows 5.5 volts freshly charged BUT with only a 2 amp load - it sinks to 3.9 volts- which means it has very minimal ability to deal wit h even small micro servos which are the popular digitals of 40 in ounch torque - these draw 300ma each just in casual use
I am certain the vast majority of unexplained "failures ar e simply voltage depression.
FWIW