I have one main report with inside many subreport, one of this seem need always to be in new page.
In this way, if the previous subreport exeed the first page ,also only 1 o few lines, all the space remaining in page are blank, and the next subreport start in a new page.
I need to collapse all, to have no more blank space in the page.
Sounds like a group option "StartOnNewPage" or perhaps "MinHeightToStartNewPage" or "RunToBottom".... so multiple reasons could cause your behaviour
Without your JRXMLs and a screenShot of your problem, it's difficult to analyse and post a solution