We have an ice farm and there is a single block that does not form ice ever. It is the same biome (mountains), same y-level (96) and the same light level (sky 15, block 0) as the adjoining blocks. All blocks have full sky access. The block in question is not on a chunk border. Water is not flowing.
I don't see this reported anywhere. It's a subtle thing, so may being missed.
I am not sure what I could provide to help troubleshoot this.
A screenshot would help.