just a few opinions on my behalf...
with plyometrics, it's impossible to be far from beginner and not be able to jump as high or higher than your standing vertical jump, when dropping from box heights of say 12-18".... because that is failing to achieve the basic concept of plyometrics.
this is paraphrased from jumping into plyometrics:
determining depth jump height:
1. athlete is measured as accurately as possible for standing jump and reach
2. athlete performs a depth jump from an 18 inch box, trying to obtain the same jump & reach score
3. if the athlete successfully achieves the task, box height may be increased. box height should be increased by 6 inch measurements. step 2 is repeated until the athlete is unable to obtain the standing jump and reach height, this box then becomes the maximum depth drop height.
4. if the athlete cannot reach the standing jump and reach height from an 18 inch box, either lower the box height or abandon depth jumps in favor of strength training.
in every case i've seen, clients who couldn't reach their standing vert after dropping from a box had these issues:
- extreme coordination issues, poor ability to maintain center of gravity
- lack of unilateral strength, poor performance on db stepups, db reverse lunges, db SL squat variations etc
- unfavorable body composition
- unfavorable conditioning (GPP)
- weak ankle joints (unable to maintain dorsi flexion during various agilities/sprint work, and stay on the ball of the foot)
i'm not saying ANY of that pertains to you, i'm just telling you what i've seen. maybe it will help, who knows.
what i've seen that helps alleviate those problems include:
- improving general agilities & "agility"
- improving single leg agilities
- improving body composition
- improving unilateral strength
- introducing low level plyometric exercises (variations of mini reactive hops, double leg, on to 3-6" boxes)
- introducing low-level (~6" box) depth jumps over small hurdles
- emphasizing ankle locking (dorsiflexion) mechanics, speed, and relaxation for pretty much everything just mentioned
also, i've never seen anyone proficient at the agility exercises i prescribe, be unable to perform reactive exercises at a moderate to high level of ability.
as far as single leg depth jumps go, they are almost pointless if you are unable to perform proper high intensity double leg depth jumps. this is because the movement will become one of strength, not reactive ability. the amortization phase on the ground will be far beyond that which is necessary to induce proper neuromuscular / physiological changes in the body.
when it comes to single leg plyometrics, something that involves consecutive single leg hops would be much more appropriate. here, the body can utilize reflexes (cross flexion / extension) to produce greater power, while still overloading the single leg tremendously.
here is a very high intensity variation:
performing that at lower intensity (while still providing benefit), would just require changing the cone distances (making them much smaller), or doing it without cones and not going for max effort.
i wouldn't do that though unless you're drop jumping properly, and single leg (strength/agility) is at an appropriate level..
anyway, i type too much