Dear Chris,

Sounds like it might be a wetting problem.  I don't know any surfactants that
survive HFN.  You could try a DI water soak, with no dry, for a few minutes
just before HFN.  It is also possible that the offending channel(s) have a
slightly different surface oxide, a quick 10:1 HF dip, with or without rinse
just before HFN might help.  If 10:1 makes it worse, then chemically oxidize
the surface with nitric before HFN.  The idea is to make all the etched
surfaces start the same.

Let me know if this helps.

Best

Dean