DIY yeast CO2 problem

Hey, all,

Just wanted to see if anyone else out there has had this problem and might be
able to help. I have a 120 gallon tank with DIY CO2 injected into the filter
pickup. Problem is, my CO2 production is extremely low... Like 1 bubble per 3
minutes. The first batch I ran on this tank kicked butt to the tune of a
bubble every 2 seconds for two weeks, but to date all successive batches have
been duds. I tried adding baking soda, but it didn't help at all. In fact,
that batch was probably the worst to date. I cant seem to get this stuff going
well. I use a 1 gallon gatorade jug with 3.5 cups of sugar and half a tbs of
Fleishmann's active dry yeast. Can anyone figure out what's causing this
problem?

Thanks in advance!

Matt