It is a bit intimidating making comments on technique in Alex's tribe, but here goes anyway.

1. Do not despair, you will get it !
2. Practice more often for shorter times to build up embouchure "strength". My experience is that once you get tired additional practice does not help develop your embouchure. I like mornings and evenings. Even if you only play for 5-10 minutes in the morning, it will help your body to remember what it is supposed to do.
3. Think of having a direct, open passage (including your throat) from your diaphragm all the way up and then down through the kaval. This means that you should face forward, head up, and have your air passage as straight as possible. Move your arms around to position the kaval so you can make this happen.
4. Have the kaval as straight out in front of you (not to the side) as you can manage with your chin up and nose pointing straight-ish ahead.
The second player in this video is a good example:
www.youtube.com/watch
(please note, some of the best kaval players don't look anything like what I am describing)
5. Why body position? It is the air support that will drive the "wind" which is the source of the kaba sound. Keep in mind that *everything* must eventually be loose and relaxed, arms, shoulders, fingers, lips, mouth, throat... that may take a while. Use a mirror, preferably a full-length one that you can stand in front of.

I will make some suggestions on lips, mouth, and throat in another posting.

Good luck !

...john