Yes, that's normal and necessary for thermal expansion reasons. The main body 'floats' on the four screws threaded into the core. Don't force it.
I'm not sure what good repeating this advice is to the OP. I'd suggested CO2 oils, "Have you tried other oil? Fairly thin CO2 oils seem to work very reliably." but they are far from necessary, other types can (and do) work well if they do well on the foil test. It's good to remember that only a fraction of us have access to CO2 oils, and unlike BHO, QWISO, Hexane and other techniques the cost/complexity of rhe rig prevents isolated individuals from making their own.
Good to try if you can get them, generally excellent extracts I think, but not necessary to proper use/enjoyment.
OF