Thats a shame, I didnt think you would have a problem this soon.
Is it binding because of the long distance of the y compared to the truck spacing or is it just the weight causing excesive friction? If its the former then it should disapear if you drive it with a screw on each side. (eliminate the racking) might be worth pursuing before for you make major changes.

For the rails I beleive Gary at Zapp sells the 15mm rails and 4 way loading trucks for the same as the Igus stuff you bought. £50-60 for 1m rail and £35 ish a truck. These also have way more capacity as well.