My Pt cruiser 2.2L Diesel is giving a hard time to start

I have bought new spark plugs, a new battery. And
there is a new air filter installed in. The fuel injectors
have been tested to see if they are producing diesel
and they are working. The starter solenoid is
working perfectly. And the diesel pump is working
pretty fine. But if you crank the car, it keeps on
cracking and won’t start. Some times if u manually
heat up the engine by switching the ignition on and
off for about 7- 12 times then if you start up the
ignition the engine might start-up. Any ideas what
may be the other problem causing this hard start?