Overlaps in rdlc report in wpf

i have 6 reports in my project, in wpf where i am using MDI container
i am using user controls for showing reports or for containing reportviewer
when i open report 1 it shows properly,
now when i open report 2 or ANY OTHER window or user control without closing first report, user control or window (of second report) goes behind 1st report viewer and rest on  1st user control

means the sequence is usercontrol1, usercontrol2, reportviewer1, reportviewer2 when report 2 is active
but when report 1 is active, the sequence is usercontrol2, usercontrol1, reportviewer1, reportviewer2
.