FarmBot Express - wheels flared slightly

I now have my two FarmBot Express 1.0 (+ 25cm extrusion added to the Gantry) for my slightly wider beds. Everything is working. However, the wheels on the side that “glide” on the sides of the garden bed are flared out slightly on both sides of both units.I This causes the unit to occasionally fall off the sides despite numerous attempts at adjustments. I am wondering if some shims could be added to where the sides connect to the gantry to correct this? Any other suggestions?
P.S. Since I have two FBEs side by side I did a bit of experimenting. My raised bed is several years old and made with cedar. The top of the edges is no longer smooth and has some raised grain lines. I used a rasp to reduce them; but they are pretty straight and if anything should help. On one of the beds I added a layer of nylon tape to try and improve the smoothness. However, the falling off problem is worse for this bed. But might be better if the wheels ran parallel to the side boards.

@dlmcshan Could we see some pictures of the beds and the problem you are experiencing?

Attached are some quick photos (starting to rain). (Could only include one picture.) As you can see the “flair” out is just a few degrees but after 1 meter along in that direction sometimes takes it close to the edge. The other side is not so bad. The views of the two FarmBots shows about the same results.