Attaching shockleader

Every time I try to attach shock leader the mainline snaps when I apply some pressure. Any ideas what might be wrong.

http://www.netknots.com/fishing_knots/alberto-knot

Does your main line snap if you attach a swivel or hook directly on to it?

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had that problem before what line is it and how old? bin it tho if can snap under tension of a knot will snap under tension of a good fish

This has happened to me before. It's your mainline, either it's cheap stuff or it was left exposed to sunlight/the elements for a good while. When I first used shockleaders it took me an hour to realise that I needed quality mainline to tie it. The knots you read off the internet are usually all reliable but if you have mainline thats not in good shape then they won't work.
I've used this knot for months and it's worked perfectly, I've used off the ground cast, overhead thump, the Brighton cast and attempted (when there was no-one in my vicinity, may I add) the pendulum cast and I've never had a problem
http://www.gofishing.co.uk/Sea-Angler/S ... ader-Knot/

The Alberto knot is nice for lure fishing, once it's kept outside the rod eyes when casting. This makes is good for 3 to 5 foot length leaders, maybe a little more if using a longer lure rod. The reason for having the knot outside the rod eyes is that there is a small tag facing towards the casting direction. This tag will bump the guides on the way out and eventually lead knot failure.
The knot on the go fishing site is good for General beach casting.

Alternatively, try using the "Lefty Kreh Leader Knot":
http://youtube.com/watch?v=GDxGy3Nld2g
Easy to tie with similar line material or with different line-to-line materials. You can tie this one even with your eyes closed or with frozen fingers!

The Lefty Kreh knot looks good. It would be more for joining different thickness of Mono or possibly Fluro for fly fishing. Normally I buy tapered leaders now for dry fly fishing, but I'll give this one a try.