Curious if you change anything as you shoot a group of skaters. Are you going with f/2.8 always or stop down to get more of them in focus?
Shooting freestyle groups can be tough, especially if the choreography has each skater doing photogenic elements in unison. I look for portions of the choreography that gives each skater a moment to shine alone. If you don't know the routine, it helps to be able to shoot with both eyes open so you can tell when another skater is preparing for a jump or another element. Still, you're going to miss a bunch of shots. For these groups, I keep it wide open and I'm primarily looking for individual shots.
Synchro is different and I will shoot wide open and stopped down to 4.5, maybe 5.6. If shooting from the boards, I like to get a shot wide open, looking down the line as they skate by with only the second or third skater in focus. If the line is skating toward me and perpendicular to my focal plane, I'll go wide open because shutter speed is more important here. If I try to get more skaters from a line in focus, stopping down means a slower shutter speed (or higher ISO) and I make sure to pan with the movement as I shoot. This helps if the skaters are moving at the same rate and not in a pinwheel.
When shooting competitions, I generally can't shoot from the boards so I forget about the more dramatic shots of looking down the line. Actually, I am more likely to shoot from the top of the bleachers with synchro so I can show the synchro elements as the judges see them. Hopefully, this means straight lines! The key is to find a point where you can look over the plexiglass and to be on the same side as the judges. Most routines are choreographed to face the side of the rink where the judges sit. In this case, I typically stop down to 4.5 or 5.6 and leave it there.
One nice plus to competitions is that you may get the chance to shoot in a rink used by a large university with LOTS of light. The shots in the gallery link below were shot at the home rink of the Minnesota Gophers hockey team. It was a dream to shoot with this much light. These shots were at ISO 4000, f4.5, and 1/640.http://www.skatetheoval.com/page/show/281573-photo-gallery