Recently bought a used SMT10 to get back into RC. Swapped in an Onyx brushless motor and esc that came as a combo. Now I’m having trouble getting reverse to work. Brake works and reverse was fine with the stock setup. It will eventually work now and works flawlessly at times. What did I do wrong?

did you re-calibrate the new ESC to your radio?

No, I didn't. I assumed it was a bonehead newbie mistake. My first time swapping out a motor, or really anything other than replacing a broken plastic part on an RC car. Thanks

unlikely you did nothing wrong. I think its all in trigger control. esc wont let you just go into reverse.