Happened to me, and many others, in the first few weeks of ownership. After having it happen only a few times, it stopped. Seems that it just needs a little time to move past this occurrence as things get broken in.
On my 2017 the rear hatch fully closes and latches with very little effort provided I have opened one of the Volt's doors or windows (part of the way open.) Otherwise the Volt cabin's air seal is so tight that the rear hatch does not fully close and latch.