...I suspect your problem is coupling.
I have found that if I do not get the loop in the clear, it is hard to get good swr figures.
It does not need to be high off the ground (I am assuming you are using it vertically), but must be away from conductive materials.
Another quirk I have noticed with the MFJ magloop is that its capacitor/motor drive system really needs a super-slow-motion setting.
You may think you have gotten to the bottom of the swr curve, but I am betting that if you give the up/down button a quick press you can get lower.
I concur on both points.
Point 1, coupling: A small magnetic loop seems to couple into its immediate environment very closely. If you have a lot of wiring, metal ducting, or (my case) big cast iron radiators nearby, it won't tune up anything like it will in the comparative freespace of a suburban back yard.
Point 2, tuning: The biggest surprise for me in fooling around with mag loops has been how amazingly touchy the tuning is, especially at the bottom of its frequency range. My homebrew 31" diameter loop goes into and out of tune on 20 meters over a capacitor movement of 2/3 of a millimeter.
One more thought: if you're running 100 watts while you're sitting close to the antenna, you're running too much power.
Good luck, have fun, and I hope you get to move into a more ham-friendly place soon.