Obviously, the more difficult crosswords have fewer entries and - assuming you get them right, of course! - you have a greater chance of winning. In the past few years, I've won a Mephisto and a Spectator twelve times each, an Azed four times and a Guardian Prize once. Still no Listener win, though.
In fact, I went through a phase a year or two ago when I started to get pee'd off if I went six weeks or so WITHOUT a cheque, dictionary or whatever coming through the door!
Go on submitting is my advice.