My heart rate monitor typically doesn’t register on any device I’ve used (including my Garmin 520) until I’ve warmed up for a few minutes. It’s particularly an issue when I work out inside in the winter on a trainer.
This normally isn’t a big issue as my Garmin will just pick up my heart rate monitor shortly after I begin my ride. My problem with Zwift is that Zwift doesn’t seem to try to pair with my heart rate monitor once I’ve left the pairing screen. So, I go the whole ride with no heart rate and as soon as I’m done with my ride and go back to the menu, I see that it’s now reconnected.
I’ve been using a Scosche Rhythm+ for a couple of years now - easily the best HRM I’ve had including those from Wahoo and Garmin, chest strap and optical. With all the chest strap versions I just dipped the whole thing, strap and hrm, in warm water then put it on - worked fine then. Completely unnecessary with the Rhythm+ though, it connects and transmits data virtually soon as I switch it on.
It’s an optical hrm and comes with a wrist sized band and a larger one for the arm - I use the larger one high above the elbow so it sits inside the sleeve of a short sleeve jersey, so well hidden from view. I find that the best position, after a bit of experimentation, to get a really consistent value with my older chest straps. I can’t fault it at all, and its much more comfortable to wear than a chest strap.
For years I’ve used electrode gel on my HR strap before I put it on. One tube has lasted me two years, it’s cheap and effective. Only a little bit is needed. You can find it on Amazon or maybe at a medical supply store. (I snagged my tube from work)