Settling behind block retaining wall

I have a 4' tall interlocking block retaining wall in my yard that is about 3 years old. It's situated on a steeply sloped part of the yard so as to provide a flat area for a jungle gym, picnic table, etc. Since I've owned the house (about 2 years now) I've noticed that the back-filled area behind and above the wall has been consistently settling or is being washed away. Is this type of settling/erosion something I need to be concerned with and correct? If so, what is the recommended course of action to correct it? Do I simply need to add more back-fill or is the solution more complex than that. The wall was installed prior to me buying the house so some of the installation details are unknown to me. I do know that a perforated corrugated pipe drain was installed behind it since I can see where it exits on one end of the wall.