Slow start?

Hello,

When starting my diesel, it doesn’t fire up like normal. It takes maybe about 7 seconds of cranking before it fires up. If I go to the store or somewhere and come out to start the truck, it fires up right away after about 1 or 2 seconds of cranking.

What would be the cause for it not to start up right away when the engine is cold?

Thank you so much, y’all rock!