I had similar problems with two MD holsters I tried for my Glocks.
One was too loose, to the point I could easily shake my gun out. The other too tight, to the point where holstering and drawing the weapon took more attention than they should, and safety became a concern.
I ended up sending both back for credit.
I have been happy using Ky-Tac kydex holsters for my Glocks, in both belt slide and IWB configurations.
Live Free, or Die