NTFS has some additional, hidden metadata it usually puts around the middle of a partition. It, in my experience, can't be moved around except by some proprietary software.
The now-defunct Partition Magic could move the MFT around, and a quick search shows that this may be able to move it around, too. There's a trial version available, so perhaps that might work.
There's also MyDefrag, a highly configurable disk defragmenter app that provides some facilities for defragmenting the MFT. In my experience, I could not succeed in getting it to move all of the metadata preventing my resize, but perhaps it'll work for you.
Having dealt with this hassle before, though, I would strongly advise that you consider backing up this partition's data and re-creating it anew, if only to save time and many a headache.