It is possible you are unable to edit certain sessions of the workout plan because the member already validated these sessions. You can only edit/modify the sessions which have not been validated/completed by the member, double check what session your member is on and try again. You can see what session the member is on by going to the "Home" menu and selecting "Fitness KPI" from the drop down menu.
Then click on the "Communication" tab. Here you will see a list of all the members that are assigned to you. You can search by name. You will also see how many sessions have been completed (also letting you know the next session you can edit. For example (see image below), John Doe has completed two sessions, so I can edit the third session and onward, if needed.