i'm on my way to getting a 120cm dish to work with my hh120 for ku-band only.
I was having fits for the last couple weeks because I could only get 24 deg of arc. Based on a hunch and the fact the pole looked like it was bent slightly, I purchased a pole level and found my pole was indeed out of plumb, thus confirming most likely why I wasn't getting the whole arc.
I have to put in a new pole -- probably the j-mast that came with the dish -- and better pole supports (and I recommend you use great supports, too) because a 1.2m dish is extremely front heavy. I had a 90cm on the setup working good before the changeout to the 120cm and I also replaced a light cheap chinese LNBF with an heavy invacom, so it looks like the added weight of the upgrades finally was able to pull the mounting pole out of plumb whereas the 90cm was light enough.