I recommend a number of things:
1. Be sure the dancers all know the counts, or if you're just having the "feel" the music -- which is fine, too -- then the are sufficiently familiar with it.
2. Stop and correct as often as you need. Don't wait until the entire piece passes, give notes, and then run the whole thing again. It's actually more efficient to stop and correct as you go.
3. Check to ensure the dancers are initiating and completing the movements at the same time. While they may all agree as to where the downbeat is, they may individually interpret slightly. [Part of achieving unison, is common training -- like from the same ballet school. This is why this is important.]
4. Try rehearsing your piece in chunks.
5. Be firm about where you want the counts and emphasis is to be. Things can change in the blink of an eye, without anyone intentionally meaning to make any changes at all.
6. Have them run the dance without the music. The dance should be able to stand independently of the score.
Hope some of these helps!