From the manual
Note: “Forward/Reverse with Brake” mode uses “double‐tap” method to engage reverse, i.e.
when you first apply brakes (The 1st“tap”), the ESC begins to brake and the motor reduces speed,
but it will not engage reverse. When the throttle is moved back to neutral and quickly back to
brake, (The 2nd“tap”) reverse will engage as long as the car is stopped. The “Double‐tap” method
can prevent mistakenly reversing if the brakes are frequently used.
So the car must be at a standstill for it to engauge reverse.
Also try putting brake power at %100 and re-calibrate your rx to the esc. I run %50 brake on my Reventon's and they stop very well