I just bought an M3K but haven't installed an extended tube yet. It seems strange that yours ran fine for awhile. Is it possible you have switched brands of shells? I can't see any reason for the spring to suddenly start binding. Maybe you just need to replace the spring. I just watched a Jesse Tischauser vid where he left 20" of spring beyond the end and it functioned fine. Good luck with it and please continue to post your results.
https://www.youtube.com/watch?v=hl1JyGzjzUg