I use 18-20lbs main with a 60lbs shock and i manage to get a neat knott, when you're loading the shock on you want a least half a rod length of shock from the rod tip to the top of the rig body then 5/6 turns on you're reel.....does this make sense?
Anyway if you're cracking off whilst you are "loading" the rod during the cast it could be the knott or a nik in the shock
If you're cracking off after you've released the cast it's more likely due to over-run on the reel causing the line to flare and snap off - try smoothing the cast or using more brake on the reel to slow the spool - when you get more confident you can do this with you're thumb
Anyway if you're cracking off whilst you are "loading" the rod during the cast it could be the knott or a nik in the shock
If you're cracking off after you've released the cast it's more likely due to over-run on the reel causing the line to flare and snap off - try smoothing the cast or using more brake on the reel to slow the spool - when you get more confident you can do this with you're thumb