hi my answer look like silly, but have you format the data by any chance, if you didn't then try it after flashing the custom rom, i too have this issue but now after install custom rom i wipe the data each time. logic is simple go to wipe option then click on the option format data and type yes then click enter button.Ok guys hope someone can help. I had havoc installed everything was fine, I then installed Kang rom and everything was fine except I couldn't get root access. I wasn't aware that I needed to use canary build of magisk. So I decided to go back to havoc. I used the one plus tool to restore factory rom/firmware. I then flashed twrp,IMG. And zip latest. I flashed magisk 20.4 then I flashed havoc. But now Everytime I flash a custom rom it doesn't take. It says successful but when I reboot it goes back to twrp I switch slots then the factory rom boots. I flashed it on both slots but a never takes and b is always factory rom. What am I doing wrong I'm sure it's something I've overlooked. Not too familiar with a/b devices. Thanks for any help. Also I've downloaded different rooms and tried them as well but all do the same thing. I downloaded files two or three times thinking I had a bad download but it's always the same. Anyway if you guys can help I would be very grateful thanks.