my truck wont start.

i have a 2005 Cummins and im having a hard time starting this cold season
never had a problem before. i have changed the fuel filter on it and us the
ps fuel additive and it still either start after turning key for a while or it wont.
when i do get it on it run perfect no issues. if i pull the fuel cap off and fill it
with diesel it usually starts i dont think its getting the fuel it needs to start.i
have had the truck for about 50,000 miles and i have never changed the
fuel pump before. like i said when it on drives great with no issues its just
getting the darn thing on this season and is my fuel pump located on the
block or in the tank i dont be leave its on the block havent noticed anything
would help guys its my work truck and ive been driving my girls jetta tdi to
work lately by the way thats an awesome car little noisy but run great.
neways any help would be greatly appreciated.