[Ralph]

The high pressure pump can supply very high pressures but not a lot of volume,
if one or more of your injectors have excessive leak back the pump wont
be able to get the pressure high enough to start the engine, have a look
on u-tube for diesel leak back test and think about doing that. If it is one or more
injectors this will also lead to air in the return lines though there is always a little.

 

[Ralph]

It may be effected by pressure but you are looking for uneven returns
one or two injectors returning more than the others.

 

[Ralph]

It may be best to get them cleaned/serviced by a diesel specialist and ask if there
is any sigh of metal contamination, if there is then it may be the high
pressure pump is on the way out or if you don't fancy that get a new filter
and cut the old one open and see if you can find any metal particles in there.