Service not selected in drop-down list


I’ve got a problem with selecting the right services in the following page:

Actually in my settings I have several services, some of them last 90 minutes and other last 30 minutes. In this page I want customer to book services delivered by a specific service provider which are only services which last 30 minutes. I specified the specific service provider in both the service drop-down list and in the chalendar.

As you can see the drop down list shows two services, which are both 30 minutes long. But the chalendar shows the time-table for services lasting 90 minutes. If you manually select one service from the drop-down the chalendar correctly change showing 30minutes appointments.

I could not see a short-code option to set a default value for the service.

Can you help me solving the problem ?