When you login to the PBA Reseller Control Panel from the Provider CP usign the 'To Control Panel' link in a reseller account properties you do not get privileges of the 'Full Access' reseller's role.
If you login to the PBA Reseller CP using the 'To Control Panel' link on a reseller's account general screen in PBA you get logged in under the active reseller's staff member (user) with the lowest ID above 1000000.
So, you get logged into the Reseller CP with privileges of the user which may not have the 'Full Access' role assigned. The user under which you are logged in can be found at the System Information > Personal Info screen.
Log into the PBA Reseller Control Panel under a specific user which definitely has the 'Full Access' role assigned.
From the POA Provider Control Panel it can be done at Operations Director > Reseller Manager > Resellers > select a reseller > General > Staff Members > press the 'Login' link under some user with full access role.
From the PBA Provider Control Panel it can be done at Operation Director > Account Manager > Resellers > select a reseller account > Users tab > select a user with full access role > press the 'To Control Panel' link.
There is a request PBA-27919 to implement possibility of changing the default user which will be used when clicking the 'To Control Panel' link.