For more on setting up rules see this article

Your diary will most likely have rules set up that maximise your diary.

A common one is no 2s on a 4 top - meaning you will not short sell your 4 tops. 

However, there are days when you might want to disable this rule (i.e. valentine's day, special events or day when all your 2 tops are booked). 

How to disable a rule for a particular Day/Service

1) Go to the diary date you wish to disable

2) Click on the spanner icon  located above the book button

3) Click on Rules Exceptions:

4) This opens up a window with all your rules:

This will disable the rule for this diary date only. 

For more on rules see this article