How to rewind line on to strimmer head?

Hi

Forgive me if this a no-brainer.

I cannot rewind new line onto my black and decker electric strimmer.
Its the type where you stop and start the motor to feed the line.

It has a detachable red plastic cover over the line spooler. When this is
removed the spooler just 
pulls off. The spooler has 2 separate channels where one winds each piece
of line, and a couple of 
notches to hold the end of each piece line when it has been wound.

When you replace the spooler in the machine the line feeds through 2 metal
"gates" - the part the 
protrudes does the cutting.


I am clearly missing the plot because when I wind on new line and then use
it it lasts for @[EMAIL PROTECTED]
 30 secs 
and then does not strim. When I look at the spooler the line has either
withdrawn inside the spooler 
completely (i.e. not sticking through the "gate" and hence is not
available for cutting) or in a 
tangled mess around the spooler.


Is there a trick to this? How tight does one have to wind the line?

Any help is much appreciated before I throw it away in frustration!



Thanks - Adam