How do I thread the nylon cord?

Every time I thread the nylon cord of my strimmer, it fails as soon as I start using it and I find the thread has once again been lost in the bobbin!

I'm taking a few inches from each side and fixing to the slit in the bobbin's side so it's firmly anchored.

I'm feeding thru the hole on each side but when I switch on the cord breaks and its all retreated back in bobbin. What am I doing wrong?