I've never heard of pre-soaking a diffuser - I've always just put them straight in. I've tried two Do!Aqua ones in the past, and some cheaper options. All of them have been OK.
I'll leave the new diffuser sitting in the tank today, and connect it up tonight to check it again. Thanks for the suggestion.
I left the DOOA diffuser to soak for 24 hours, and tried it again this morning, leaving it to run for a couple of hours.
It's far inferior to my previous Do!Aqua diffuser, non-branded glass diffuser and even the airstone. It's noisy and unusable, so I have switched back to the airstone permanently. I will not be using the DOOA diffuser an anymore. I'm a big ADA fan, but it's not a good product. I'm glad it's not so expensive here in Japan.