Assuming the bolts are tight then there could be another problem. Perhaps remove the loader and then the mounting bracket and see if maybe there is a weld fillet, spatter or similar that may be preventing the bracket from sitting flush to the tractor frame and fully torquing down. Also is there a possibility that the bolts could be shanking out, running out of threads before there is sufficient clamping force, I found a condition similar to this on my Kubota, not on the loader bracket but on the top bracket for the three point. Y'all might also consider fabricating a cross brace similar to that used on some Kubotas. Also, look for a broken or incomplete weld or missing hardware where the frame cross member that supports the frame where the loader bracket mounts. Anyway, just some thoughts. J