Flats can be tough in longer focal ratio newts and compound scopes. I used to find issues under city skies (light pollution) shooting flats without my dew protector (that had been place while shooting lights) on my newt. Sky flats (possibly with T shirt) may be your best bet if the reducer keeps doing this to you. Not terribly convenient however, particularly without automation to keep varying exposure length.
If all else fails, fake it! Apply copious quantities of Pixinsight DBE!
