if anyone is still confused, do this:
deleate whatever preview button you were previously using on your form
add a new button that previews the report you want
IMMEDIATLY(dont add anything else) right click and go to the build event option, choose code builder
that should bring you to the visual basic window
scroll all the way to the bottom (thats where the code for your button should be because it was the last thing you added)
(type it exactly as it appears, that means with the " " , just change the reportname, ect... to what your stuff is actually named)
there should already be a line that says:
DoCmd.OpenReport "yourReportname", acPreview
you should add this to the end of it:
, , "[UniqueIdFieldName]= forms!yourFormName![UniqueIdFieldName]"
hence, when it is done, you will have this:
DoCmd.OpenReport "yourReportname", acPreview, , "[UniqueIdFieldName]= forms!yourFormName![UniqueIdFieldName]"
i hope that solves everything