The availability statuses (busy / maybe busy / free) of events imported from an external calendar are set by the external calendar they originate from. The status cannot be changed within Howbout unless the events are converted to Howbout events first. 

If the external calendar doesn't give Howbout a busy / free status, the status will default to 'maybe busy' to avoid incorrectly marking you as busy / free.

Still doesn't seem right? Check that you've correctly imported your calendars into Howbout.