"a diffusor from ebay" isn't going to cut it, there are many different kinds and i've heard that the jap plant guys' glass diffusors require more pressure than dyi can effectively provide.
what kind of piping do you use? non CO2
hose (i.e. regular air hose) dissipate 30% of the co2
into the surroundings.. maybe that (paired with a high pressure demanding diffisuor) is why there is never enough pressure to release co2
through the diffusal method.
you could try to create a stronger mix (more yeast) and see if that makes it possible to diffuse the co2