Greetings farmerdave!
Glad you found this forum too! Is your boiler a siphon or pressure system?
I dont remember the specifics on it, but I am pretty sure waste oil has more BTU's in it than home heating oil. If you are no longer running 100% waste oil and are mixing in HHO, your BTU's may have decreased. Now I'm not sure how different it would be mixing it, say, 50-50. Maybe it wouldnt even be enough to notice. I think you are on the right track with changing different nozzles. Have you noticed a decrease in fuel usage? If so, then yes, maybe the nozzle is plugging up.
Best of luck getting back to your normal output. Enjoy the forum!