Hello Gabe,
looks like a bug -- can you please open an issue report at
https://github.com/gc3-uzh-ch/elasticluster/issues ?
I am on vacation until end of June but I can take a look at it when I'm back.
Anyway: please make sure that the user or IAM profile that you are
using with ElastiCluster does have permission to access those private
AMI images. If everything is working correctly with the public ones,
my guess would be that it's just a permissions/authorization issue.
Ciao,
R