Dumb question, but have you tried using a different electret mic element ?
I had initial low output problems with my Bitx40 using a cheap computer headset with an electret mic element.
I switched to a different headset and now I am getting full power out. It seems that some electret mic elements
are more sensitive than others. I didn't see you mention anything about what you are using as a mic.
Michael VE3WMB / VA2NB