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A bit more data would help to analyse your problem Roger.
What LRGB filters did you use for mono filtered imaging? Not all filters are parfocal and not all filters are 1:1:1 combine ratio. This is one main reason I use Astrodon. If you focus on the luminance you will find your other RGB filters and even narrowband are also in focus. There may be some tiny difference but its negligible.
What camera are you using? As Peter has mentioned the Sony sensors are more than twice as sensitive than most Kodak sensors in O111 and do very well in blue. So chromatic aberration (which is often in the blue channel) will show up more than say with a less blue sensitive camera that uses a Kodak sensor.
Secondly what software do you use to callibrate and combine your images? Some combine methods work better than others.
Another point is what angles do you image at? I tend to image objects that are near the zenith or slightly to the east and then image them down to maybe 45 degrees. Below that most subs start to deteriorate in quality.
What scope is it? Does it have a corrector? Is there mirror shift? chromatic aberration?
How do you autoguide? Is there differential flexure which may get worse at certain angles?
How often do you refocus and how sensitive is your scope to temperature shifts. How much does your environment shift in temperature?
Are you imaging at a dark site or under light pollution? Is your seeing very variable or tends to be reasonably stable?
As you can see there could be several factors.
Greg.
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