I am not convinced the CO2 supermarket regulators with an in line atomisers are hit and miss, they would have been tested at 2bar and certain flow rate so not convinced a quality issue.
The pressure they produce (2-2.5 bar) is absolutely fine for in line diffuser. However from multiple experiences myself and reading here, if you have even the slightest leak in your setup, they appear to be very difficult to control bubble rate and/or fail to work with an in line diffuser. So people go out and buy an adjustable regulator crank it up to 3-4bar and suddenly in line diffuser works and bubble rate is nicely controllable as they have overcome the gas leaking out.
I have had numerous leaks in my CO2 setup (with in line diffuser), despite being a very small leak, seemed to make control of bubble rate quite difficult and variable. Bubble rate seemed also to speed up, then the gas "burps" out the diffuser and bubble rate slows down, all very confusing. Each day bubble rate would come on slightly different as well, including the odd day and yellow drop checker and "sleepy" fish...whoops. 2Kg fire extinguisher also lasting 30-40days rather that 150 odd (or more) days.
I have had leaks appear from seams of cheap Ebay bubble counters, leaks from plastic crazing on bubble counter, leaks from locking nuts of JBL bubble counter and recently from the seam of a one way valve. Despite dunking the equipment in water to check for leaks, some only appeared when pipes were moved/manipulated.
Now all sorted and leak free now (I hope). I too was convinced regulator had failed (or was cr*p) and there was no way my system was leaking.The bubble rate is easily adjusted from 1 bubble per 10 seconds to far too many per second to count (along with white out in the tank)!!. Once bubble rate is set stays set when solenoid opens each day.
So my theory and practical experience is you have a leak, if you can't get inline diffuser to work and can't set reliable bubble rate.