My engine wont crank with spark plugs in?

Recently replaced the timing chain on my 1991 Ford EB falcon. Im sure the valve timing is correct and the engine cranks over fine without spark plugs but when I put the spark plugs in the engine hardly cranks not even by hand. It seems like the compression is so good that it cant crank and the spark plugs aren't hitting the top of the pistons. What is going on.