I ran into this problem myself. I could not get the detail band to stretch, even though the text field in the band was configured to do so. The problem was with the report's Print Order property, which for some reason had been set to horizontal. When I changed it to vertical, the problem went away.
According to the JasperReports Ultimate Guide,
When filling report templates horizontally, dynamic text fields inside
the detail section no longer stretch to their entire text content,
because this might cause misalignment on the horizontal axis of
subsequent detail sections.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…