Time slot does not work for all services

As a podiatrist I offer 13 different services (see picture).
The time during the service is different, the smallest is 10min and the largest service is 50min.

The basetime is 10 minutes.

When I test the agenda. it works fine for the first service of 40min. Times where already an appointment is in Google calander, are blocked well. For example 31 okt: pnly 12.40 is still free to have an appointment, the rest of the day is not possible.

But when I try another service for example 20min, then the whole day is available, which is strange because only 12.40 is still available.

What is going wrong here?