You've probably tried this already but mine used to often play up and I had to apply weight (i.e. sit) on the back of the hatch whilst pushing the button...
Go into the car and remove the trim panel (held on by trim clips unless you have speaker in there) and lever the catch manually, it's an electro-magnet and the chromed pistons require at least cleaning (and kept free of grease etc.) or re-chroming. Adrian@
I went a step further with mine, leaving the trim panel off and using a length of broom handle to pop the hatch, which saved me having to keep climbing over the seats.
I used the same broom handle to prop up the hatch when the gas struts got lazy.
