Adding weight to the rotor head will seriously reduce flight times, those tiny China made motors react badly even by adding a slight amount of weight! Remember these are micro/mini helicopters and are more sensitive to adding any kind of extra weight! As for your current problem take a good hard look at you control rods, arms and swashplate to see if there is unseen damage. If not any of those you might have damaged your 3-in-1 gyro/mixer/esc unit, if everything look ok its time to call e-Flite customer support.