On aphids

Awmack & Leather, Host Plant Quality and Fecundity in Herbivorous Insects, 2002.
* sap-suckers prefer high amino acid levels in phloem
* herbivores in general don't like high carbohydrate and lipid levels, because it dilutes their target nutrients. Although aphids were *more* successful under high CO2 conditions, so it might not be that simple.
* Plants produce defense compounds which can put off some insects (including aphids). This is more true of generalist feeders, whereas species-specific feeders don't seem to mind so much.

So at a guess, your plants might have *too much* nitrogen, which is why they look great but are suffering from aphids. And/or they are lacking some micronutrients which means they can't produce sufficient defense compounds. If your greenhouse is well-sealed, then elevated CO2 might favour aphids as well.

(I have seen this in brassicas. Once I started soil testing and balanced my soil, my aphid problems disappeared. Then I ran out of organic fertiliser pellets so substituted in blood and bone, and got an aphid infestation. I figure the highly soluble nitrogen from the B&B over-dosed by plants on N, making them more attractive to aphids.)
 
Powell et al, HOST PLANT SELECTION BY APHIDS: Behavioral, Evolutionary, and Applied Perspectives, 2006
* aphids use a whole range of methods to choose which plants to feed on
* during flight they are attracted to yellow wavelengths, rather than green, so are more likely to be drawn to leaves which are young, sick or senescing
* they are also attracted/repelled based on odours, but that seems to have more to do with finding the right species, rather than the health of the plant (though there are so many compounds they can sense it might be much more complex than that).
* once they get on the plant they "taste it" to get a feel for what the nutrient levels are like and the presence of defense compounds.

These reviews are getting old, though, so perhaps there is more up-to-date knowledge out there.