The hort oil should work. Insecticial soap and using a soft brush
should work too. Another method is to use a Q-tip and swab with
rubbing alcohol. For a more drastic cure, set the plant in a trash
bag, spray with malathion, and close up the bag for 2 days. Repeat
the procedure in 10 days. There are over 200 species of scale insect.
Females molt twice and remain on the host plant; the male molts twice,
pupates, then emerges as a winged-adult to find a female. Scale will
infest nearby plants, so it is best to isolate your dracaena (it may
be getting re-infected.) I had two parlor palms heavily infested
with scale, and finally got rid of them using a series of systemic
insecticide treatments.