It turns out, settlers love tatos - despite the Brotherhood's assessment that they are disgusting! This mean's they are eating and trading them instead of stockpiling them in my workshop for me to use to make adhesive. 

Solution? Mod the vegetable starch recipe to switch out the tatos for something they aren't as keen on eating - melons! On one hand, where's the starch? On the other hand, melons are very sticky. (Also, while they are neither vegetables or starchy, melons are the only growable crop that isn't used for any crafting in the base game - until now. [Edit - thanks to Streetyson for pointing out they are useful in Survival mode.])

As to why there's no tatos (or razorgrain) in my workshop, I point to the work of John Smith Games, and this video showing the results of his testing of what crops get eaten first. It seems that until I have more tatos planted than I have settlers in all my linked sites, there wont be any tatos in my stores. Forums also cite a known bug where settlers won't harvest tatos which could be adding to the problem (or possibly caused by their rapacious appetite for tato fritters). 

Either way, Loopy Kay's Melon Glue is the solution for the crafty survivor. Now available at all stoves.

(For the curious, I am using Valdacil's Item Sorting which results in the tag {scrap} in my screenshot. Having tried a few sorting mods, I like the way this one groups things, e.g. keeping all left-armor pieces together.)