There are a few reasons Amy and Andrew may book over a meeting on your calendar.
If you’re on a Google calendar you should first check to see if the conflicting event(s) on your calendar is set to “available.” Amy and Andrew see your calendar in black and white, you’re either available or you’re not, and they see an event set to “available” as a cue that it’s ok to schedule over.
For Google calendars, there’s an option to make an event either “available” or “busy.”
For Microsoft calendars, you can list an event as “Free” to let Amy know she can schedule over it. “Tentative,” “Working elsewhere,” “Away,” and “Busy” options will all appear busy to Amy, and she will avoid these slots unless you tell her otherwise.
Second, if you add a conflicting event after Amy or Andrew has sent the invite, they will not automatically reschedule their meeting. Once Amy and Andrew add an event to your calendar, they will leave it there until you or your guest ask to reschedule it via email.
Finally, you may not have connected all of the calendars that Amy and Andrew should be using to determine your availability. You can check your connected calendars here and make sure Amy and Andrew have access to the ones they need.