My wife and I had the same problem (having to redo HRV readings multiple times; sometimes 30 min before we got a result). We have two units, and through a process of elimination, we figured out that one of the Polar straps was faulty. Polar took care of it through their warranty service. No problems since it was replaced (2 months or so now).
FYI - if you have the ANDROID version and must use the earphone adapter, I've not had any luck getting though a valid 2:30 hrv reading while having the earphones plugged in. :^( Not much help in hearing the GONG when it's unplugged, but the unit works perfect without it.
This morning had to do my HRV reading four (4) times before I was able to complete one without too many errors. Yesterday was a heavy lifting day for me and my back was spasming while I lied down to do my reading. Everytime I had a spasm I noticed a Pulse and RR error (they went RED). Just sharing as I thougt it was interesting how the back spasms impacted the readings or at least that was my observation. My HRV dropped six (6) points putting me in AMBER status.