The goals of the scissor maneuver is to get your lift vector behind the bandits wing line. As noted controlling your forward velocity is key to this, and can be done by both use of the throttle, flaps, and lift vector orientation. So how do we tell if we're losing the battle to control wing line, we look for keys in the position of the other aircraft. For instance, if we notice the craft moving towards our aft during our reversal we are likely losing the scissors battle. Next, we maybe losing the scissors if the enemy is transitioning through it's reversals fast than you. That is, the bandit is the first to roll out of a reversal and roll back into you. These are all pretty good indications that you are losing the scissors battle.
To correct the issue you have two options. 1, Run; get in phase with the bandits via a lead turn and attempt to split S out of the scissor via the bandits blind side. 2, attempt to reduce your forward velocity and get your lift vector at least on the bandit and possibly behind. Typically this would be done by reducing throttle and dropping flaps. Though, if we're early in the scissors and have sufficient E, we could attempt to reduce our forward velocity via a barrel roll and blending some rudder. To put our lift vector in the bandits high 6. Though if the bandit pulls up into us we have now begun a rolling scissors.