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Peter,
Flat frames with narrowband filters require longer exposures and/or turning up the brightness of your light source. There's no other magic to it. You should be registering some signal from a long exposure unless your light source is very dim.
How long have you tried? What are you using for the light source?
I normally use a LED light box. I run it dimmed for LRGB and at full power for narrowband. My narrowband exposures are usually 10-20 seconds.
Cheers,
Rick.
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