Often times, basketball players have used one-leg jumping their whole life. It's just a much more natural movement because it's used every single time someone goes for a layup. On the other hand, volleyball players are often used to two-foot jumping because it is the most common way to jump when trying to block or spike. So, if you have all the suppositions to be a great two-foot jumper, but you get barely of the ground this way, it's probably because of a lack of technique.
The simplest method to measure an athlete's vertical jump is to get the athlete to reach up against a flat wall, with a flat surface under his/her feet (such as a gym floor or concrete) and record the highest point he/she can reach flat-footed (the height of this point from the ground is referred to as "standing reach"); fingertips powdered with chalk can facilitate the determination of points touched on the wall. The athlete then makes an effort to jump up with the goal of touching the highest point on the wall that he or she can reach; the athlete can perform these jumps as many times as needed. The height of the highest point the athlete touches is recorded. The difference between this height and the standing reach is the athlete's vertical jump.