This seems likely to be an inventory issue more than anything else. Fine-grained targeting requires fine-grained ads which requires a sub-linearly scaling sales team.
It's not something you'll hear unless you're in the know, but there is a pretty high level of animosity towards Pinterest in most retail marketing departments. I love Pinterest, so it was surprising to learn when we started this work, but it boils down to: 1) traffic from Pinterest ads show very poor quality / engagement. 2) Pinterest has not been a good partner - they've actively "snubbed" a number of big labels. There are even a few major retailers who "deleted their Pinterest". 3) Organic Pinterest basically doesn't work, so they won't even put an intern part time onto managing their Pinterest account. Even one of their top retailers, Nordstrom, appears to use a bot to post to their account.
When your ad dollars don't show ROI, people spend elsewhere. And if your main customers aren't engaging your platform, that's definitely trouble too.
So yeah, a ton of fear mongering on here. I'm long on Pinterest.