AWS.config.credentials are null between page requests

I'm using Facebook to create an authenticated identity via AWS Cognito, that's all working fine and I can login, and synchronise data.

However, if I navigate away from my sign-in page - but remain in my site - the underlying AWS.config.credentials object is then null and I can't synchronise any data via a different page.

Suspect I'm missing something obvious but can't see it from the Amazon docs and don't know what!

Edit: Sorry - should have added - this is via the Javascript SDK