Not all my fields are showing in my dropdowns (and those that are aren't all correct!)

I'm using this module to show a list of species, and a list of taxonomic species groups.

So, I have a group that is, for example, all the dragonflies, and a list of Latin names for each species. The idea is to stop people from accidentally classifying something as,say, both a species of dragonfly and an amphibian, so they have to specify it's a dragonfly first before they can say it's Aeshna isosceles.

I installed the module, but had a few hiccups because annoyingly, neither of my fields are unique. The setup assumes that the second 'postcode' field will always be unique, but this isn't the case for my dataset. I have to deal with a plant that is classed as both a conifer and a flowering plant, for example (dratted thing!) so I need to be able to have duplicate items in both fields.

So, I fiddled with the database and made a new autonumbering key field, so I could import my data without causing Mysql to barf on my duplicate fields.

So far so good, but alas, now the selector doesn't seem to quite work. It sort of does - if you request dragonflies, you get 5-6 dragonflies, and 2-3 butterflies, but not all of either.

I appreciate this is a bit outside the original scope, but I wondered if you could give me any hints on why this might be happening, where to start with debugging?

Many thanks for any help that may be available!