Difficulty truing wheel

I have been building wheels for years but can't seem to true the rear one of a pair I built recently. The front couldn't be any straighter but the rear is giving me greif. All I want to know is if it's worth re-starting it again from the point where the last thread is just about visible.

I've never had a problem building a wheel; it is a simple process as long as you read up on what you're doing, get advice off experienced wheel builders, take your time and concentrate. Maybe the fact that nothing has ever gone wrong is the problem here as I don't know what to do when something does go wrong!

Obviously all of the spokes are the correct length, have had the same number of turns before being dished, spoke paths were set, stress was releived etc. But Is it worth gradually loosening all them to the point where the thread is just visible at the end of the nipple, and starting again? If I had made a slight mistake earlier would going back and starting again fix it? Or is it possible that the rim wasn't perfectly straight?

I'm mindful of the fact that the rear rim turned up in its box with no bubble wrap (the front was well protected and came in a seperate box) and there were a couple of minor scratches on it. I was a bit worried that this indicated that something may have happened to it. I wanted to send it back or take it back to my local branch (you may be able to guess where I ordered it from) but decided not to bother as I didn't want to wait any longer, and when placed on a couple of flat surfaces (windows) it appeared to be perfectly flat... then again who is to say that windows are flat...

Will dismantle, clean the grease off the eyelets and try and get a refund if it doesn't play the second time...