New Steelworxx Sliding Down the Shaft

I just got a new Steelworxx revenge model, but it shifts down roughly 1.5 cm along my shaft as I walk around. The A-ring measures 45mm with a 35mm tube and is 100mm long. It doesn’t appear too large or small for the ring itself, yet I’m considering whether a smaller size would be better. Any advice? Aside from that issue, I’m very happy with it—the build quality is excellent and far more comfortable than my CB6000.