Lights

Might be the angle but looks like the back band grip is too thin, also I'd have the band catch go higher and cut away more so it sits more snuggley. Maybe like a cut out where you expect the band to go down the sides. You'll also benefit from something tacky/with high friction on the contact side of the mount.

But your comment about weight might be right. The exposure set I have are very small and light, maybe it's only so good because the lights are so light.

@andy_k has the right idea... Although I'd have to have dedicated night polo, night mtb, night track, night baby bike /tourer/cx bike...