This sounds exactly like an 8602 situation.
I suppose the more the power, the less the sensitivity because of extra noise, ie declined reception.
I only find the SR2 and the NL-2511MP to have higher power and good sensitivity. That gives us another conclusion, better quality equipment.
The SR2 has the best receiving with -98 db sensitivity.
Also, the NMP-8602's production quality is so bad that even individual cards differ in performance from one another, and at least 1 out of 10 new cards is bad!
Yet it is cheap and worth the trial and error factor to get a good card out of a bunch.