I've had my Venu Sq for about 2 months now and have noticed the same sleep tracking problems.
1) Watch often doesn't begin registering sleep until long after it should. For example I go to bed at midnight and fall asleep normally (10-15 minutes) but watch registers sleep beginning at 2:30 pm. Sometimes it registers correctly but most nights it doesn't.
2) Watch doesn't record any Deep Sleep. In two months there have been 1 or 2 nights with deep sleep.
I talked to support, they looked at my stats and said that these problems were caused because my watch was measuring too high a heart rate (75 bpm) during my sleep time . When I look at the stats my watch is registering higher heart rate than my typical resting rate (60 bpm) that I see when I'm up but relaxing.
Anyone else having this kind of problem or have any suggestions as to a fix.