Reporting API - Segmentation not showing all results

Here is my query string to the api:
?module=API&method=Events.getAction&secondaryDimension=eventName&segment=eventName==banner_321&idSite=27&period=month&date=last12&flat=0&format=JSON&filter_limit=-1&token_auth=AUTH_TOKEN

I’m only receiving data for the last two months. But when I browse the events via the piwik dashboard ui I have data for the last nine months. Any idea what I’m doing wrong? I added the filter_limit=-1 but that does not change my result.

The question string, translated to English reads as: ‘get me all event actions for banner 321 for the last twelve months’.

Thanks for the help!