Hard to push
I've experienced that with a few different vacuums. Does dis-engaging the drive cause the front to lower itself? You may have to either raise or lower the machine a notch or so when using or not using the self propelled function. They can feel clunky when used without the drive due to the extra hardware installed also.
With motor off you can flip the machine over and observe how the mechanics work while switching back and forth with the lever or whatever engages the drive. Good luck. If you can get your hands on a handbook or download one, there may be clue in there.
