Honey,
I suspect that the bearing that the wheels ride on is wore out.. See Item 18 below (Odd, it show item 22 until you point to it and them it becomes item 18..)
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Also, just to make sure.. The threads can only go on one way.. There is a notch or groove that the new thread must fit in. Pretty obvious, so I assume you put them on correctly.
If this were my robot I'd order two wheel kits..
Thanks,
Jim R.