I had a similar problem with my Vauxhall Movano van after I mistakenly chose the wrong fuel filter of 2 (went for the smaller of the 2 as the other looked too big to fit into housing), and it wouldn't start like it use to. The fuel was going straight back to the tank and I had to crank it for England till the filter filled up again & it would then start! I ended up putting a squeezy stress-ball hand primer in the pipeline to prime it 1st to ensure it would start 1st go. This worked for a while till the squeezy was no more and became rock hard! Bit the bullet went for the too large fuel filter & it did fit (DOH!!), and hey presto!, starts on the button even when left for days. If you've changed the fuel filter, check & check again. And when you've checked again, check it again as that might be all that's causing your prob.
Ian