Front brakes act differently from rear brakes. When you apply brakes the weight shifts forward lowering the traction of the rear wheel and increasing that of the front.

That's why auto brakes are proportioned with over 60% of the brake force applied to the front wheels. Even then cars are prone to rear wheel lockup.

Likewise on a bike, so locking the wheel is a poor way to judge whether brakes are efficient. Instead try to judge if you're getting sufficient and similar slowing through mid-range braking. Remember that rear stopping power is self limiting, but front can increase to the tipping point at which you see an endo.