First of all, just for info, screeners do not have the option of giving warnings - they come from Johan alone.
Secondly, the number of pictures recieved by A.net every day is tremendous. Unfortunately the screeners do not get to see any reports stating how many are recieved, but I would not be surprised if it was averaging close to 500 (maybe more on weekends) per day. In response, Johan appears to be making his criteria even more strict - with so many uploads, he can afford to select only the caviar and filet mignon. Our target as screeners is to minimize the number of shots that get past us and that are subsequently rejected by Johan. But he has a tendancy to surprise us on that fairly often - a shot that may have been acceptable a month ago may now be unacceptable. This is in keeping with Airliners.net's aim of being the repository of the best pictures available.
About your pictures, you said yourself that your local developer does a so-so job. So at least you are ahead of some people who simply cannot see any fault (or refuse to see them).
I think the reason for the warnings is that the lighting in the pictures appears over-corrected - i.e. unnatural. You mention that the pictures were taken near sunset. The placement of the shadows and the low lighting angles confirm this. But the luminosity is so bright as to be in the middle of the day. Shots taken at this time of year in England should be quite cool, like this:
Click for large version
Photo © Chris Coduto
The problem with prints is that the developer will often "correct" away the sunset lighting effect. And if you do have an uncorrected scan, selecting the automatic color adjustment which every photo processing software seems to have will also wipe out the nice sunset lighting, and turn it into "daylight". I don't know if your developer did it on the CD (I don't know if they usually do that, like for prits), or if you did it, but at some point your pictures got hit with this automatic color adjustment.
Also the pictures appear to have been brightened too much. It's as if in addition to broad daylight (caused by the color correction), you had also brought along some huge 1000 MW flashbulbs!
The overbrightening, along with the color correction, leads to the unnatural lighting effect, which tends to wash out detail. Not quite enough to cause rejection for the pics you posted, but enough to be clearly not as good as it could have been - hence the warning.
On other notes, your pics look good. The Hertz plane in particular is just about perfect in terms of sharpness without getting jagged (slight hints onder the rear of the fuselage, but not a big deal). The hazy sky could be better, but you can't do much about that.
You might ask your developer to transfer your pictures to CD without any color or light processing at all (which is what I recommend to anyone using print film anyway). You might also consider switching to slide film, for which CD scans are also available, and those that I have seen do not have any such "corrections", since I suppose they know that slide shooters are generally not interested in that part of the process.
I hope that helps a little.