If a station has moderate to severe multipath interference, that will cause the HD component of the broadcast signal to drop out, especially if the multipath is severe.
Also, note that the HD signal on many stations is 1% of the primary FM carrier, with some stations running 4 or 6%. 10% is the maximum, and in the U.S., requires additional authorization. I can't speak for Canada. If a given station has no adjacent stations either side of it then 10% could be doable.
DAB listener 12/2003-11/2011 (DAB Off-Air since 11/2011)
HD radio Listener since 06/2010