Have tried swapping the other reverser??? If it works fine, then its the reverser. If not...............................It could be the servo, the Rx, the Tx, the programming, sub trim, trim, expo, d/r, length of control rod is a 1/4 turn too much, too short, ATV or something complicated. Been there done that.

Any chance you can do it in the radio and not use the reverser? Use a matchbox?