This is what the documentation says:
The plugin takes into account only the files updated since the build start (determined by means of the last modification file timestamp).
There are two ways to setup report processing. Through the build features or through service messages.
If you do it through service messages you can set parseOutOfDate='true'
to ignore the timestamp on the file.
##teamcity[importData type='typeID' path='<path>' parseOutOfDate='true']