The fact remains that the surface if MDF is consolidated with a wax like substance..I've never had a problem gluing MDF face to face...... BUT I usually sand everyting prior to assembly.
Ya dont have to rough it up as such but a light surface sand may make a difference.
check too that you have shaken your PVA and that it is niether getting stale nor been subject to near freezing temperatures.
if you realy want strength and perhaps a but faster setting time step up from the white PVAs and into the yellow PVA alaphalactic resin glues like AV syntec AV180, triton yellow glue or similar.
cheers
__________________ Any thing with sharp teeth eats meat. 
Most powertools have sharp teeth. 
People are made of meat. 
Abrasives can be just as dangerous as a blade.....and 10 times more painfull.  | 
MDF to MDF? 
