In the past every time that issue has been seen it was the 40 low pass filter having
wrong components installed at assembly. Other wise the issue would be evident
on other bands.
Generally the power goes down as one goes up from 80 through to 10M.
Nominally 40m should be less than 80 and more than 20. This is assignable
to the power amp.
If 40 is much lower than 20 then low pass filter or switching is broken.