Hi Carol,
This is the right place to raise this question. You can always open support cases.
I suspect that this user may not have permissions to all the projects within the workspace. It may be that he was left out of one/few projects in which case he won't be able to see defects of these projects even if they are within the workspace.
The only permission that is across the entire workspace is a Workspace Admin. This permission will allow a user to administer anything within the entire workspace. Other than that users need to get permission for each project within the workspace.
This is explained here: Administer Users | CA Agile Central Help
My suggestion is you examine this user's permissions and see if they are have permissions to all projects under that workspace.
If this didn't help you resolve, then it may be best to work this through a support case. Whether you open a case or not, you may want to take a screen shot of all the permissions of this user and include so that we can examine.
Let us know if this helped resolve.
Thanks,
Sagi