Just to make sure I am reading your post right, the slant shown in the pictures goes from side to side, not front to back? Basically, I am wondering if your machine is out of tram, which means that the x-axis is tilted which might cause this issue. Especially since the difference is huge from one side to the other. I would suggest tramming this axis as shown by brent here. A note on this is that you can use soda cans instead of the blocks he used as their tolerances tend to be pretty tight.
Quick outline of the steps:
- Turn off machine
- Put cans/blocks on the base plate under both sides of the x-axis
- Physically pull/push the x-axis down to rest on the blocks/cans
- Turn on the machine (to lock the axis in place)
- Swap the blocks/cans with each other. If they slide under both sides with relative ease, then the tolerances were good enough for this process and you are finished