I suppose it depends what you've tried.
How large is your party?
How flexible are you with the timing of your meal?
One thing that I have found is that playing with the timing of your request can help. IF you say you want 7pm, then that available 5pm table might tno be showing up - even if 5pm isn't your ideal time and the online sytem says it's showing you 'all times available from 5pm to 9pm.' Try puttign in a request for 5:30pm if you've been saying 7pm - you might get lucky that way.
Also try looking at other dates within your visit.
If all else fails, book some place different. 'Ohana is good, but there's so much to see at WDW, I'd only notice it a tiny bit if we didn't eat there. you might even be able to get a table at Kona next door.
If all that fails.....where are you staying? onsite?