You mic output might be bad. I had this occur on my bitx40. Over the course of a couple weeks I was not getting more than a watt out.?
You didn't mention the power out when driven by the computer.?
On my ubitx, made a 317 powered regulator and run the rig from a 20v old laptop power supply Warms the clouds to the tune of 20 watts on one swr meter and 30 watts on my older power meter.?
The case on my bitx40 is too small to add in a similar power supply.?
_Dave_ AD0B