If you have a digital multimeter, check the amps on the 9v bateries.
Even if you use rechargeable AA bateries, they come with different amps.
Easy way: Use a baterry craddle as advised before (just for the sake of learning, measure the amps using the craddle, so you can have some grounds to compare).
Best way (includes a lot more, chargers) Lipo battery, the ones used in RC cars, airplanes.