Delegate to Mailbox unable to edit meeting responses

Hi, 

i have a delegate to a senior mailbox user who has reported that they cannot edit any meeting responses to this mailbox in the tracking details of the meeting.

they are able to edit their own meeting tracking responses for their own mailbox, so it must be something related to permissions on the delegated mailbox, but not seeing where? 

using W10/Office2016/cloud environment/desktop install

any thoughts?

Thanks