Is it possible that this is the issue for you (from the developer doc):
Pagination
Search results are not stable; repeating the same query multiple times may return the data in a different order, even if the data do not change. Because of this, the traditional pagination available elsewhere in the Asana API is not available here. However, you can paginate manually by sorting the search results by their creation time and then modifying each subsequent query to exclude data you have already seen. Page sizes are limited to a maximum of 100 items, and can be specified by the limit query parameter.