Our scheduler uses an algorithm to ensure that appointments are booked closely together. The system needs time to learn. This is especially the case when you start using the online scheduler.
If your appointments are set to last 20 or 30 minutes, it's important to minimize other durations. This means that there should be few or no appointments of other durations (like 5 or 15 minutes) in your calendar, and in your settings for bookable services. Otherwise, the system will not learn properly.