Homebrew PCBs

Tom
  It's good to have a lot of pressure, to some extent.  I think the 
problem you are hitting now is that you are applying so much pressure 
that you are distorting the top plate and lossing contact as the top
deforms.  So increasing force on the plate may be decreasing the 
pressure where you want it.
I would try (as others have talked about) 
1) Increase the temp, 300F is about 150 C, which is low end of good.
Crank it up, 350 is still less than 180 C.
2) Back off on the pressure, and put more, stiffer, heat conducting 
plate (Al for steel) on the top and bottom (I say both because I'm 
not sure where your heater is). 
As an experiment, you might try laying out a grid of dots on paper
and fuse it to one of the sheets of aluminun you are using, try it
with a lot of pressure, and also on the other side with less 
pressure.  Check to see if it holds better on the edges than in
middle.  If it does, you might want to look at some type of curved
surface, convex up, highest in the center, with which to support
your boards.  I bet laminate glue is much more forgiving of
pressure than the toner is.
Good luck,
RM