most diy co2 problems are leaks.
If you have followed the directions in that link to the letter, I can almost guarentee you have leaks occuring where those screw in barbs are located. Possibly also at the check-valve joints. I learnt very quickly that the more joins you have in the system, the more likely you are to have leaks.
Using the "slightly smaller than the inside diameter of your hose" method, drill holes in the lid of your gas separator and soda bottles. Jam the hose in place, pulling thru with needle nose pliers. Delete all other unnecessary fittings (you can use a T-piece to connect the two bottles if you like). Others will say otherwise, but IMHO, you do not need check valves. Even if a siphon is created in your tank, it will only drain enough to fill the bottles. Being sealed containers, THEY WILL NOT, CAN NOT, OVERFLOW.
Mix, quick and simple:
2 cups sugar (white, brown, or brindle, makes no real difference)
2 cups cold water, 1 cup hot water, 1 cup cold water. In that order.
Shake, allow to settle.
1/2 tsp Yeast (some like Champagne or Brewers yeast, I just make sure I have fresh Bakers yeast. The choice is yours)
Takes me three minutes to make this mix. Bubbles within 2 hours.
Note, if you are alternating refilling your bottles each week, get hold of one or two of those little airline clamps and isolate the to-be-filled bottle from the rest of the system. This will ensure pressure remains in the system (and CO2 in the tank) until the new mix is up and going. I leave this clamp in place until the just-filled bottle is hard with pressure, before removing.