Perplexed conundrum. IR from one cam on, other is not. Or is it?

I have a Dahua HDW5231R-ZE 2MP cam doing side of house 20' up over watch duty. I have a Dahua B5442T-AS as 8' high low height duty catching the same area.
This recent (as of 1 week ago) issue is night and IR. When I view the 5231 at night, it's pitch black...as in, no IR light visible shining anywhere. However, when I view the 5442, I can see the 5442's IR light and the 5231 IR light. I have verified this by turning on/off the 5231 IR light while watching the 5442. Yes, it blinks on and off from the topside of the 5th wheel. I have factory reset the 5231, no joy. Very perplexing.
I bought these 5231's from Andy as my first 4 camera's last year. I have not checked if any firmware is available to upgrade. My other 3x5231's work AOK with IR lighting.

Holbs said:

When I view the 5231 at night, it's pitch black...as in, no IR light visible shining anywhere.

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The IR cut filter might be stuck on.

alastairstevenson said:

The IR cut filter might be stuck on.

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resolution: whack it with a rubber mallet?

Holbs said:

resolution: whack it with a rubber mallet?

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If a tap is going to maybe even temporarily unstick it, I think it needs to be a short sharp tap from something fairly hard and light such as the handle of a knife or fork.
2 lots at right angles to each other, in the same plane as the front of the camera, to get the right direction.
An IR Cut is a small delicate device a bit like a butterfly's wings.

thanks. I'll give it a go, someday. Luckily, it's an overwatch camera so IR is not vital since I have a 2nd camera 8' above.

Holbs said:

thanks. I'll give it a go, someday. Luckily, it's an overwatch camera so IR is not vital since I have a 2nd camera 8' above.

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Worth trying also is to use the camera web GUI to switch between day and night mode a few times, then back to the normal setting.