first dont use locktite on your prop nut.
secondly what type of wrench are you using,not a multi pourpose,hobby type,the one for glow plugs,a box end wrench will give you the best leverage.
also check to see if the nut is bottoming out on the threads of the propshaft.
if it is not then really crank it down tightly,the nut and propshaft is hardened steel it can take some tourque.
if it is then the prop will not tighten, and you just need to add a washer as a added spacer on the nut side, so the nut will have enough threads to bite and tighten properly.
you can also use a double jamb nut like on 4stroke engines but usually a properly tightened single nut will do for almost all 2st engines.