If you've used bearings, and you're feeling brave, you can resolve this with a soldering iron. Put the wagon on a flat surface, identify which bearing needs to be adjusted down (you only need to adjust one side of one axle) and then remove the relevant wheelset. Warm the iron and then touch it to the bearing, applying gentle pressure in the desired direction until the bearing just starts to move in the softened plastic. Remove the iron immediately and assess whether the wheels are now closer to level. The plastic will reharden quickly so you can have another go if things need further adjustment. Don't dwell with the iron or the plastic will melt all the way through to the front of the axlebox. It's a method of last resort but I've found it works if done with care! I had to do it on on one or two Dapol cement wagons with the soapy plastic.