Why am I not able to access another partner account for a user?

As a partner user, you only get access to your own Partner accounts. Hence, you aren't able to access another partner account for a user.

However, it depends on your configuration. Here's how you can check the user specific access:

1. Click on the Settings and Actions menu and go to Setup and maintenance.
2. Search for the Manage Sales and Services Access task.

   The Sales and Service Access Management page appears.

3. Click on the Explore Access tab and fill the required fields.

Now you can see the accesses related to the selected object.