In nearly all cases of this the issue is that the customer is using the iOS event data inside of Informant and the device is set to only show 30 days or so of the event data.

To correct this please do the following-

Go into the device settings (Not Informant)>Calendars. Scroll down to the bottom of the settings page.

There you will see a header for "sync", under that header you can change the range of data that your device is syncing/showing.  By broadening the range of the synced data your data should display more than just a month.

Once the changes are saved it will take few seconds for the data to be visible in Informant.   

**This only works if the source data is still available to the device**