It just needs more time. Give it about 6 weeks, that should help. Carbonation can take longer than you might have heard, and longer than you might think.
Keep it warm, don't chill it until you are ready to drink. I agree with brewchez on that.
Sound like its just not done yet. Wild yeast will work slower than some purer strains will work. If there is pressure under the cap when you open it then there is some activity going. Of course, if there wasn't enough sugar then maybe you've just got enough CO2 in the headspace. I think more time will be needed.
The notion that the liquid only absorbs ...