Worked on my 640 with Windows 10 fine until mid-may or so. Probably whatever update occurred then was the issue, but whatever it was--notifications have almost stopped entirely, though sometimes alarms come in hours later on my band.
Reset phone, band, removed health app and reinstalled. Went to the "Fast preview" build instead of standard. Nothing.
Really getting annoyed--the most important thing to me is receiving notifications on the band. MS really needs to get its act together, as even their biggest cheerleaders (me) aren't going to stick around or recommend their stuff if it is this unreliable.