We have already purchased extended support for Win7
I doubt this statement as this is not an actual offering; thus I assume that you've purchased Extended Security Updates (ESUs) which is not support in any way as all Win 7 support ended one year ago.
Having said that, we have not in any way explicitly stopped the ConfigMgr client agent from installing on Win 7 systems and to my knowledge, this does still work fine and many orgs (unfortunately) are still managing their remaining Win 7 systems successfully with ConfigMgr.
Without any technical details though, nothing more can be said here. Specifically, have you reviewed the ccmsetup.log on the systems having an issue?