have you spoken to the good folks at rockland colloid about your problem?
i have used LL off and on since 1986, expired, new plainold LL, VC as well as AG+
on glass and paper and never really experienced what you are talking about ...
but i never used an oil based urethane pretty much everything else under the sun as a sub though.
maybe .... the oil in the urethane is somehow fouling your gel layer and damaging your emulsion?
have you used the emulsion using a different sub layer ... like gelatin?
maybe bob at RC can help, he knows his stuff !