laguna diesel wont start without a jump?

my 1.9 laguna diesel will only start with a jump off another car, or a battery booster. I have had a brand new battery on it, just put brand new glow plugs in it, and its the same. If its warm it will start on its own as it should. I runs fine, no missing or flat spots.

When turning the key on a cold morning, the yellow light comes on as it should, then goes out, then I try to start and the light flashes and the engine just turns without firing. It may fire once for a split second on the first attempt, but it dies and the light flashes.

Its strange how it will start with the help of another battery.

any answers?

thanks