We've had this happen with both of our dds. In dd12's instance, she didn't actually show negative growth, but her math RIT score in 4th grade from the fall to the spring went up something like three points when the average was more like 10.
That year, she was placed in a regular 4th grade math class with "enrichment" that entailed me pulling her and any other kids who had scored 80% or better on the pretest for the unit out once/week to do things like suduko puzzles. Dd was the only kid in the class who scored an 80% or better on the pretest for every single unit. Her reading RIT was in the 99th and stayed in the 99th. She makes progress in reading regardless of instruction. For math her RIT in the fall was in the 97th and in the spring was in the 91st.
In her case, I could clearly say that it was due to inadequate instruction that year. She skipped 5th grade the next year and I feel that her math instruction has been adequately challenging since. Math is not her strongest subject so 1 yr of acceleration has been sufficient.
In dd10's instance, her RIT scores have fluctuated wildly. Both math and reading have been somewhere between the 50s to 90s with scores varying by as much as 40 percentiles from the spring to the following fall or vice versa. In her case, I have to say that I don't think that the scores are a very accurate representation of what she knows. We have a lot of theories as to why they move around so much, but I don't really think that it is due to instruction.
