The Idler wheels despite being considered the non adjustable side, when not initially tightened in place are able to move somewhat on the base plate.  If the idler wheels are initially set too tight or too loose then the Adjustable wheels may have insufficient range of movement to achieve your required rolling tension.  Subsequently your extrusion may be too tight (even not able to be inserted at all) or is too loose

So for initial setup of the Idler wheels

1. Lightly loosen the lock nuts for both Adjustable wheels pulley wheels
2. Set both Adjustable wheels to the widest setting and tighten the Adjustable wheel lock nuts
3. Loosen the idler wheels lock nuts just enough so the idler wheel assemblies can move around in the plate mount hole
4. Run the gantry on to some of your target 2020, 2040 etc extrusion
5. Using your hand lightly clamp/squeeze the Idler wheels and Adjustable wheels together
6. While clamped, tighten the Idler wheel lock nuts
7. If may take a couple of goes to get a feel for this, but so long as it is not initially too tight final rolling tension can be set using the Adjustable wheels