If you want to redeem HA miles for Delta flights, you need Delta's Low band award inventory.
http://dmn.delta.com/skymiles/direct/charts/us49/
The complaint that airlines don't make enough Low/saver/off-peak awards available is universal across FlyerTalk, but Delta's profoundly dysfunctional award calendar also make those available awards difficult to find. Advance search techniques are found in the thread below:
http://www.flyertalk.com/forum/delta...ss-awards.html
Hawaiian Airlines reservation agents may not have the patience to conduct extensive searches for Delta award inventory; certainly some of Delta's own are unwilling.
Instead of trying to master Delta's award search, why not fly a different Hawaiian partner carrier? Maybe Virgin Atlantic, Miami to London (or beyond)?