Full access to hierarchy list (selective access) and WSA access but occasionally will get No Access

n0200309

I am supporting a model that has selective access to a hierarchy list called Department Hierarchy and Top level called ALL Department. For Model Role, I have full access and for the Department Hierarchy Write access, it is All Departments. I am also a WPA. Occasionally, I will get a message that says Department Hierarchy: No access. Why is that? What would be the reasons? Would it be because too many users are using the dashboard and hierarchy at the same time? There are about 145 users in total. Does anyone have any ideas?  It was not working for a period of time yesterday and this morning, I have access again. The 'No access' to the hierarchy is causing disruption to business users who have full access but cannot access it. I have no explanation to give to the business and this doesn't look good on our team. Any help or suggestion would be very much appreciated!!! Thank you!