[AccessD] problem sorting in group

I've got a simple report grouped by a date field using the Week Group On 
setting. However, the dates within each group aren't sorting, even though 
I've set an Ascending sort both via the Sorting and Grouping feature and in 
the underlying query.

Year values are the same, but only one group is sorting as expected -- 17 
July, 18 July, 19 July, and so on. All the others are just skipping 
around -- 19 July, 18 July, 15 July, 16 July, 17 July.

I don't understand why the dates are sorting correctly within the groups.

Susan H.