@minnapple - thanks for the pointer!
However, the advice received from Google support (change max. CPC to "manual" and ensure all products are targetted, then wait) hasn't helped (so far...)
What I'd like to achieve is to use my existing campaigns (thus incurring no extra daily costs) to advertise a small number of products on Google Shopping. I have 65 products split across two (existing) campaigns, each product (and variant) targeted with "adwords_labels" in the Merchant feed and Adwords. Surely this would be enough? Can anyone offer any insight?