The biggest issues with any varnish no matter the finish, is humidity and temperature. Secondly, many model varnishes are lacquer based and dry almost instantly when in contact with main air flow in an airbrush. Lastly, if all else is correct, apply two or three light coats 9-12 inches from model and then ensure the model is immediately left to dry in warm, dry environment.
The Humbrol varnishes are ok, even Army Painter products too. Revell do a nice selection, and Dullcote is still fine as a flat. Oddly, many manufacturers advertise a matte varnish and it has a distinctive sheen, more an 'eggshell' sheen, so the definition of semi/matte etc seems to be interpreted differently by different makers.