Aerobics classes use a specialized set of choreography, with a specialized vocabulary. A number of choreography sequences are there in aerobics.
In professionally created fitness music the songs are designed to support and help the instructor. The songs are created with 8 beat or 32 count "phrases." If you were marching, every footfall is a beat.
Alternating feet as marching does, you would step 4 times on each foot in an eight count. The foot that steps first is called the lead foot. For a well balanced class you should alternate which foot is doing the leading on your moves.
The first beat (that the lead foot starts) on is called the "downbeat." The second beat is an "up beat." In an eight count there are 4 downbeats and 4 upbeats. Some choreography has the main action on the down beat, and other choreography has the main action on the up beat.
An example of the upbeat choreography is a kick or knee lift. To kick with your right leg you first have to step onto your left leg. The physical action is step-KICK, whereas the march on the downbeat is STEP-step.
Pay attention to your music and see if some songs fit better with different types of choreography. See if you can pick out the downbeat and upbeat and see if thats why some moves fit and some don`t.