If you look in Help, About Microsoft Outlook, Disabled Items is your addin
listed there? If it is then Outlook is disabling it when it starts up. You
can highlight it and click on Enable and see if that changes things.

When there is a disabled add-in, Outlook creates
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Resiliency\Disabled
Items.
It adds a binary value for that add-in with a name that is randomly
generated. The Resiliency key exists if there is at least one disabled item,
but if you re-enable the add-in then the Resiliency key and DisabledItems
subkey are both deleted. So the presence of the Resiliency key serves as a
general test for the existence of disabled items. You can re-enable the
add-in by deleting the specific binary reg value, or by removing the whole
key.