Not a lot to go on from the picture and result of the query but...
Your hierarchy seems to suggest YQMD. From the fact you get a result suggests that you have added the week between M and D. As weeks do not fall nicely into months you have needed to specify the month as part of the key for week. On weeks that all completely with a month this will return the correct result. For weeks that span month end then the formula "Week" will return only the days of the week in that month.
Sept 31,2013 is Y2013. Q3. M09. W40. D20130931
Oct 01, 2013 is Y2013. Q4. M10. W40. D20131001
When PPS processes the WEEK formula, it find the date first e.g. Oct 03, 2013 then finds it's parent for week. This returns Y2013. Q4. M10. W40. This week shows the aggregation of its children but does not include Sept 31 as this belongs to a different branch of the tree.
To fix this you would need to use a different data connection pointing at a different hierarchy in the cube with either YWD or YQWD (although this may have the same issue)
Hope this helps