The query looks OK. However my initial guess is that the work item doesn't fall within the range you specified.
Please be aware that:
- StartDate & DueDate are internally stored as DateTime (meaning, time of the day IS significant).
- Times specified in API calls are in UTC.
For example, if you want to capture work items starting on May 28 or sooner and your organizational time zone is PST (UTC-8), the correct condition for StartDate would be StartDate < 2015-05-29T08:00.
Please try it out.
If you still cannot figure out why some work items are left out, I'll need your full credentials (user name & password) to have access to your account (grant login access link is not enough for the API). My email is email@example.com.
I hope this helps,