pressure has to build up in the line first, and there needs to be a pretty good amount of pressure before the bubbles are going to make their way through the ceramic. my co2 line runs ~20 feet on my pressurized setup and i use a glass/ceramic diffuser. it takes about 5 minutes before the bubbles begin to make their way through the diffuser, and i run it at 4 to 5 bubbles/sec.
diy produces bubbles much much slower than this, one bubble to several seconds. and to hold the co2 pressure you cant have any leaks where the bottle cap joins the tubing, or else the co2 will escape here instead of overcoming the pressure of the diffuser disk. try and blow into the diffuser. submerge the disk. try to make it bubble... see how much pressure you need now? this is why it is so critical to silicone the tubing to the bottle cap, and to do a really good job. Do this; take an empty 2 liter bottle. (not empty but full of air) unscrew the cap from one of your co2 mixture bottles and submerge the empty bottle in a bucket full of water, or just dunk it in the tank. then squeeze it. this is one way you can check for leaks at the bottle cap. hold the empty bottle in a squeezed state for a little bit. tiny bubbles should come out of your diffuser if there are no leaks.
if your system is free of leaks then you can relax, as it is only a matter of time before enough pressure builds up to overcome the diffuser.