Account segments have not been defined error

I recently added a few new pledge subtypes to our General Ledger Segment Mappings. But whenever I try to add a payment to a pledge that uses one of those new subtypes, I get an error.

The text of the error message is:

Data could not be saved.
In account system "<Our account system>", account segments have not been defined for the segment values: Segment 'Pledge Sub Type': Pledge sub type: <Not used>

But when I navigate to Admin>General ledger setup>Our account system>Define transaction mappings>Development>Pledges, there is a mapping for “all pledge subtypes”.

Does anyone have ideas about how to troubleshoot this problem? I'm wading through a stack of SQL to figure out what is happening at a low level, so I can work up from there. But I have a feeling there is a checkbox somewhere that I forgot to check.