There is a known issue on Facebook's end where page permissions are not being granted automatically for new Facebook Pages and Instagram Business Profiles when they're given admin access.
(This specifically applies for users who have added a Facebook page before October 2018.)
Option 1 - Currently the only way to workaround is to remove access from within Facebook and do a Facebook page bulk re-connection (If they are all being managed within the same Sendible account) or reconnect individually for Instagram Business profiles.
However, if you're managing various Facebook Pages and Instagram Business Profiles split between various Sendible accounts, this can be very time-consuming.
Option 2 - We recommend using a Facebook account that has never been used with Sendible to add to be able to add new Facebook Pages.
Facebook has gotten back to us with regards to this and said that they will only release a fix for this at the end of Q4 of 2019 as they are having an API code freeze until the new year.
We can only apologize for the inconvenience this may be causing you in the meantime.
Should you have any questions or concerns with regards to this please feel free to submit a support ticket.