Hatching is not as easy as it sounds! Can you put a bowl of water in the bator? Also adding a few new sponges may help? I have had three or four at one time, leave one in the bowl, leaning on the side so it is pulling water from the bowl, but monitor once they start hatching, as they can get into the bowl of water and drown.
Was your temp stable throughout the hatch? Did it ever go down? If so, could be why your hatch is late, your 12/31 should be hatching by now, but is not uncommon with temperature dips that they will be late. Right, you will not see anything in the developed eggs at this time, just mostly dark with the air sac visable. Those that look clear either did not develope or quit this late in the hatch. Could also be that the eggs were not fertilized, it happens this time of year with roosters, they are just not at full speed yet, even though they are doing the deed, they still are not fertilizing the eggs.