megaflow immersion

hello again
just doing two immersions on 2 megaflows
first one was a bit of a toughie.
already drained down megaflows and picked up new immersions and removed old ones.
first megaflow wot a bugger to get it in and positioned correctly but eventually did it so then followed instructions on pack and used the new backnut provided tightened up and then started refilling megaflow few minutes later ******* out through immersion pushed on and found nut didn't reach surface of megaflo so stopped supply and drained down then used existing back nut tightened it checked it refilled megaflow and hey presto no leaks. Why say to use new backnut when not fit to do the job?
megaflow no2 did as before but using existing backnut filled system and checked leaks so started to pack up my drain just incase for this scenario about 5mins or so then was about to connect electric supply when i found a leak on on backnut not major just a drip but enough for me to drain drown.
what would have caused this tried new nut but as before didn't reach to washer is in place
what can i do?