You need a JIS (Japanese Industrial Standard) screwdriver for most camera type screws.
Looseness is often caused when machine tools used to make the threads are not at precise tolerances , mostly when new (not worn out), IMO.
Be careful to use the proper threadlocker. Purple is recommended for small screws, e.g. eyeglass screws. I can’t find it on the Loctite site for some reason, though. I would recommend letting it dry thoroughly before attaching TC to lens to prevent any possibly fogging of glass from vapors of drying product.
http://henkeladhesivesna.com/blog/when- ... eadlocker/
http://www.tooldex.com/loctite-222ms-22 ... urple.html
Don’t overtighten to prevent stripping threads or shearing screw off.
Joe