Hmmm... without seeing what you are doing, it sounds like the hoop is continuing down to your body because it's losing the momentum of the spin. Maybe try an experiment - do your vertical elbow pass, but instead of "catching" with your second elbow, just stick it into the hoop without moving it, and watch what happens. It'll just "flop" and has no swing left to it immediately as it makes contact. Is that what's going on with your horizontal? Remember you have to not only have your elbow there to catch it, but you need to shift your weight over in your feet and give the hoop a good "push" with that second elbow to keep it spinning. So really concentrate on not only sticking your elbow in to get the timing right, but sticking it in and then shoving it to catch and swing the hoop to continue its path.
If that isn't what is happening, maybe you could take a quick video to share.. we might be able to help you out a bit more if we can see what's going on :)
It is a very fast move. I know some one mentioned how they thought they were bending over too much but my problem was I wasn't bending down low enough. I would bend at my shoulders but really, to be successful at this move, for me, I need to bend at the waist.
From what you said, I'm thinking you hoop to the left. I'm the same, and for me, I realize I have to be very snappy from when I first hook it on to my left elbow and pass it to my right. For me, the timing isn't so much like vertical elbow hooping where it seems much slower, and I can have the hoop rotate a couple of time around my elbow, pass it, let it rotate there... It's more like a "hi ya!" karate speed. If that makes any sense!
Also, you say you've been practicing for a couple of days? For me it took a few months to figure out! I guess it depends on how much you practice, and if that is the only move you are practicing.
This trick is easiest to learn if you start it on your knees. You can fold over far enough to really get the motion for your arms/elbows, and then once you stand up you'll have the feeling. Also, if you hoop to the left, practice starting the hoop in lasso on your right hand and dropping it onto your right elbow. Then Pass it around to the left, and back to the right.
Hope that helps! If I have time I will post a video to help after work.