I have found the online tool to be very unreliable. In some cases the overview matrix shows availability for DL for 12.5K points, however when selecting the days only 25K fares are available for instance.
Unticking the 'Flexible' option only seems to allow to jump to days which show as 'unavailable' in the matrix, and still select flights. That might be something you could experiment with. Still its a guessing game though..
Also I noticed that the DL awards have some crazy routing in some cases. For example SFO-NYC, which I imagine to be a popular route. Only offered as DL award with stopover in Minneapolis and Detroit or LA and Detroit. Insane!
Bottom line...the booking engine online seems pretty messed up. Seems to require mostly luck to be able to book DL metal.