you may wanna check how long the media has been used. if it has been used for a long time, you may have to change the whole media. ARM media will not desolve after used up.
are you using ph pen or prob to measure your chamber ph?
i guess there are a lot of air trap therefore "air-lock" in your CR. this mean, your C02 bubble count is more then the water outlet of your CR. unless your CR has the design of air re-circulation or by-pass. you may off your C02, release all the air in your CR (fill with your tank water till no more air in your CR) and re-tune again. this may help.
try not to have more c02 bubble count then your water flowrate at the outlet.