Recently had this problem myself - with only one of my lines.

These are lines original to the car, and I've removed/re-fitted them myself many times in the past with no issue.

This time, however, I'd slightly deformed line #5; bent it out of alignment ever so slightly that it didn't sit quite squarely. A little bit of tender loving care (bending) to get it right did the trick. Hint: the line ends should seat squarely; they should drop in easily, into their respective fittings with not even the slightest force to fit them. Do, this, and you'll create a good seal.

All this assuming no damage to the seat faces on either side, as others here have discussed.

I'll take this opportunity to again plug greazzer's very good work on injectors. He recently re-did mine, and the car runs beautifully.

__________________
'83 300 SD
'05 E320 4Matic
'06 BMW 530 xi
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr