Two things that you haven't mentioned, and apologies if you've done it;
1. Did you register the sub frames before integrating them?
2. Have you applied a screen transfer function - or done a histogram stretch?
The nebulosity around the Pleiades doesn't show up without a bit of stretching.
Edit - I see you've used these processes, so maybe that's not the problem then. You haven't been playing about with masks, have you?
cheers,
Andrew.
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