Not enough power in 3in1 kit

Hi all, I’m having a problem with not enough power getting to the dc motors to move the wheels of my students’ robots. We are working off of the Sunfounder 3 in 1 kit instructions 2. Move by Code — SunFounder 3in1 Kit documentation
It moves fine when wired and coded for “Move by Code” but when we code for the next project (Speed Up 3. Speed Up — SunFounder 3in1 Kit documentation), you can hear the motor controller trying to work but the wheels need a “nudge” to get going or don’t get going at all. Should we try different wiring? AA batteries with a barrel connector? When the arduino is connected to our test laptop AND the 9v it works. I know not a good idea but we were willing to try it…knowing the possible outcome.

Please provide a video of the problem to us, so that we can analyze and solve the problem.
If you have extra rechargeable 9V lithium batteries, or 9V alkaline batteries (6LR61), you can replace them and try to see the working result.