Given that the RPM gauge has been replaced with no remedy, and setting aside the possibility of two faulty RPM gauges (and lacking a verification with a stroboscope or RPM meter), this leaves the more likely possibility that you have a governor problem, or a propeller problem.
What you're describing isn't a propeller resting on the low pitch stops, but a propeller artificially held off of them. This could be due to an internal problem, such as low nitrogen pressure in the propeller dome or a limiting mechanical problem which doesn't permit full blade travel to the low pitch stops. The most simple solution is a leaking prop dome, most typically through a filler schraeder valve.
If the propeller has been stored or the aircraft not flown for a time, airworthiness directives and concerns for inspection intervals may also apply, and even a new propeller can leak down. Check the nitrogen charge.
Another potential problem, of course, is in the propeller governor. Governors themselves seldom go bad...they're very simple arrangements, but they're still mechanical devices and still have potential to fail. A weak speeder spring, stuck pilot valve (debris in the pilot valve, for example), or failed flyweight attach pivot point can cause these problems...the catch being that often you'll have immediate feedback because the propeller control will be gritty, stiff, or immovable.
If indeed you're having problems controlling RPM or can't maintain RPM, then you have a serious problem and the aircraft shouldn't be flown until this is corrected.