Cannot clone a repository via ssh

I have seen many instances of this issue in the past & have tried all the solutions proffered without any joy.
I have two projects:
One I can clone with no issues
The other fails to clone giving the error message:

git@gitlab.com: Permission denied (publickey).\r\nfatal: Could not read from remote repository

Both projects are set up using the same SSH keys which are made available via ssh-agent.
The only difference in setup is the group; they are different.
Deployment is via Ansible using the same roles; different targets.
Pushing to both projects is no issue.
Can you please suggest a course of an investigation to resolve this issue.
Would be very helpful.