Hello Mandy,
If your site is SAP integrated, you may check the Purchase Organization mapped to the user, this might be the reason since Suppliers are mapped to POrgs.
You may also check if there are catalog views configured in the system wherein it limits the views of the user based on the user groups, etc.
Hope this helps. 
You may also provide details of your site such as on-premise / On-Demand, current version for us to identify appropriate actions.
Regards,
Dash -