Problems Fixing to Plaster Board Wall with Large Cavity

[GRANT1471]

Hi All,
I am trying to fix a guitar hanger to a wall that has two layers of plasterboard on a studwork frame. I am trying to put a fitting thats inbetween the studs. So its 2 layers of plasterboard so 25mm plus 25mm stud/gap before brickwork.
I have tried to use a long plug (70-75mm) with a rigid sleeve thats specifically designed for spanning the gap. http://www.drylinepro.com/ Unfortunately after drilling the hole the screw doesnt seem to tighten just keeps turning. I think the back end of the plug is not gripping in the brickwork, possibile that I have hit a line of mortar.
Is there something I can use that can help the back of the plug bite and bridge the gap in the cavity? I thought about using expanding foam but I doubt its good enough to fix too? I was thinking that drywall adesive would be perfect but its getting it into the back of the hole would be tricky. Is the a sealant gun adhesive thats strong enough? (need one with a relatively narrow nozzle).
Simplest thing would be to move the fitting in-line with a stud, but cant be seen to be filling holes in the wifes expensive wallpaper.
Any advice much appreciated.

 

[joinerjohn]

Divorce the expensive wife, become poor and put up cheap wallpaper from B&M.
Nah seriously, have you thought about using those fixings that spread out after you push them through the hole? I'm sure 25mm of plasterboard would be strong enough to fix something to hang a guitar off.

 

[GRANT1471]

Thanks for the quick response

She has her wallpaper I have my guitars, so kinda cancel each other out.

I think they call them toggle fixings. Will have to check if they will take the load as the fitting is quite thick and the guitar hangs a fair way off the wall (bit like holding a weight at arms length) and see if I can tidy the two holes I have already drilled.
Just presumed that the strongest fixing would be in the masonry (at least partially).