diskpart has this problem sometimes, moreover, it cannot see the result intuitionisticly. If the unallocated space is in the front, you cannot extend the partition, too. Rather than under the risk of data loss, I like to use easeus partition master, which is cost-effective. The most important, it is safe and can extend the ntfs partition without reboot. you may check
http://www.partition-tool.com/easeus...out-reboot.htm