In my opinion, it's a patience thing. My ammonia seemed like it would never start dropping. I was doing 30% water changes every other day to keep my fish alive because ammonia and nitrites were both high for a couple of weeks. If I had it to do over again, I would have cycled it without fish. My best advice is to keep doing water changes so your fish don't suffer, and just hang in there. Yesterday (5 wks after setting up the tank) for the first time ammonia and nitrite were both 0ppm
As far as the 240 gal tank, I don't know how big your fish is, but I would guess that your ammonia just isn't showing up yet. I wouldn't add ammonia, maybe add another fish or 2 and keep checking....