fuel supply issue

[obsean]

my 04 connossieur,always been a good starter until diesel gets too low, then expect it to take some time and waiting ....a new development now, she wont start atall after letting fuel get down to about a quarter...have now put 10litres in tank and with the back pipe on the under bonnet filter off, i can see that its pumping through, so there is a pump working...but no starting as yet
shall i keep on trying?whats the next simple solution to look for?
Thanks for looking