nail plates do not have sebaceous glands, and so aren't oily.
HOWEVER, they could have oil on them from the surrounding skin.
BUT there aren't any sebaceous glands there either as sebaceous glands are found at the bottom of hair follicles.
If there's oil, it's from elsewhere, or touching other things...
ALTHOUGH, they can be retaining an excessive amount of moisture. This could act as a deterrent with bonding if not sufficiently dehdydrated.
Be exceptionally thorough with Prep, EXCEPTIONALLY.
Be vigorous with cleansing and really SCRUB with the scrubfresh.
If necessary, try brushing nailfresh and IMMEDIATELY upon evaporation, apply the Shellac base to that nail.
ALSO the client's nails could be simply too soft and/or she is too rough and she needs something strong.
No singular system is a 'one size fits all'. This is true of everything in life and also applies to nail enhancement products.
ie: layer of Brisa Lite smoothing gel OR
An Enhancement system such as gel or l&p.