I had a lot of problem with my core HR monitor at first also. I bought a strap like this one and since then it has been perfect for over 2 years. This strap connects a little different and it made the world of difference for me. Good luck.

https://www.amazon.com/dp/B06Y2L4NX4/ref=cm_sw_r_tw_dp_U_x_LK-2Db3YFKYKJ

I like this one from Amazon way better https://www.amazon.com/Monitor-Replacement-button-fastemers-Monitors/dp/B06Y2L4NX4/ref=sr_1_1_sspa?s=exercise-and-fitness&ie=UTF8&qid=1537898608&sr=1-1-spons&keywords=polar+chest+strap+replacement&psc=1

Many of us have had success with this strap. I’ve had mine for a bit more than 2 years. In addition to OTF I use it for all my other heart rate monitoring activities. It also salvages the battery because it’s secured by the sensor - and the only way to apply or remove the strap is by connecting the sensor. https://www.amazon.com/Monitor-Replacement-button-fastemers-Monitors/dp/B06Y2L4NX4/ref=mp_s_a_1_11?dchild=1&keywords=heart+rate+monitor+button+strap&qid=1593381656&sr=8-11