when {OBSDATE} is null create or enter some dummy date into it and the try for the same condition… when the data is null the conditions which will appy in selections and section won’t work. try creating formula as below and use that formula in group selection
if isnull({OBSDATE}) then
‘1/1/1990’
else
{OBSDATE}
if dates are null how you are using this condition in report… logically the report should not display output when data is null… as your condition is only to display those dates where maximum condition satisfies.