Keep checking!! I'd been looking for a dinner ressie at BOG for our January trip since October. Nothing, nada - until yesterday. I was able to grab BOG dinner (early, 5:10 p.m), CRT breakfast (late, 10 a.m.) and a nice dinner reservation at EPCOT on NYE.
Also, check both online and on the phone - and during normal "break times" for working people, like right before work starts (8-9 a.m.), lunch time (11-1p.m.) and before quitting time (4-5 p.m.). You'd be surprised what things start showing up then!
Keep trying - you'll get it!!