hi Luke,
one thing that might cause this is if you've got subreports and if you right click on them and choose Format Subreport > Subreport tab, that Re-import When Opening is Checked.
or you've got OLE objects on the report.
if these are not there, it may be best if you attach one of the culprit reports (change the .rpt extension to .txt first) as there's really not much to go on without a full description of everything on the report.
-jamie
