When you are invited to an event via Google calendar, you are given the choice to accept or decline. If you decline it, you may not see the event sitting in your calendar, but Google can still report it to YouCanBook.me as an existing event. That means it will block that time in your online booking page.
If your calendar availability is blocked in YouCanBook.me, you can check for declined events by updating your Google calendar settings. Under the Week menu, select Show declined events.
Make sure that is checked and then you'll be able to see the events that you have previously declined. To free your availability in YouCanBook.me, you can set declined events to "Free" or delete them from your calendar altogether.