Servo Problem with a Prosthetic Hand

Hi. I am using MG995/MG996 servos for controlling a 3D printed prosthetic hand movement using Arduino Mega. I am using a separate dedicated power supply for servos - I tried using one set of 4 to 5 rechargeable AA batteries for all 5 servos as well as one set of batteries for each servo. The battery voltage drops and keeps fluctuating and so doesn't control the hand movements reliably when servos are kind of getting stalled (fishing wire connected between servos and fingers are getting tighter) as well as at times when these servos are used for controlling the hand finger movements. This is probably happening as the stall current may be higher than what the battery can supply, but not sure why it's happening when I am using 4/5 rechargeable AA batteries for a single servo. Did anyone experience this stall torque/stall current issue or any other issues in using MG995/MG996 servos with a 3D printed prosthetic hand? Any suggestion on what can be done to resolve this issue? Thanks!

Measure the voltage - if its dipping the supply is inadequate, if not then the servos are not powerful enough for the mechanical load (and may overheat).