@john-spellman, Can you let us a bit more about how you created the key? Which option did you choose to import? Which certificate type did you use? You could try different types.
Anything you can tell us about which Identity Provider you created and what the architecture looks like will help. Is FusionAuth the IdP/SP or both?
Also, If you can share the settings of you SAML tab for the application (without sharing secrets), that may give us some insight to the issue as well, if you are using FusionAuth as an IdP.
I don't have a ton of experience with importing certificates, so if anyone out there knows better, please feel free to chime in.
I have set up a key for a SAML provider before and using an RSA/RS256 type key. I generated that key with FusionAuth, but I don't see any reason you couldn't import the key you need.
You may want to check out this blog to test a simple SAML configuration if your situation reflects the setup.