I have a 96 Nissan Sentra and I have been having overheating problems.
From my diagnosis, I determined that it was a blown head gasket.
After changing the head gasket and putting everything back together, I
can't get the car started.  I did a compression test and found that
the compression reads below 100 psi for all cylinders.   Prior to
changing the gasket, I checked the compression and they read between
130 and 140#.    

I should mention that I checked and double checked the timing of the
cam gears and the timing chain.   I checked against the shop manual
and it all checks out.   What else could I be doing wrong??  Please
help.

Al Kondo