Most likely the problem is a clogged filtration screen on the valve.
If your pressure reducing valve kinda looks like this:
Then one of the openings on the bottom of the valve will be to remove the filtration screen for cleaning, and the other will be to access the internal parts of the valve, like this:
where that "cylinder" on the left is a cylindrical wire mesh screen to filter out any dirt or crap from getting into your heating system.
It's very possible that your filtration screen is just clogged.
You can usually purchase replacement filtration screens from the manufacturer, and I know Watts used to make an overhaul kit available as a part for their pressure reducing valves.
If it turns out that the problem isn't just a clogged filtration screen, and you opt for replacing the valve, a real good way to set it up is to have a pressure gauge (first) and a ball valve (second) within a foot or two downstream of your pressure reducing valve. That way, you can close the ball valve and tighten the pressure adjusting screw. Because of the tiny amount of volume between the PRV and the ball valve that has to be pressurized, the pressure showing on the gauge will respond immediately as you turn the adjusting screw. When the gauge reads the pressure you want, tighten the locking nut on the adjusting screw and open the ball valve. The PRV will then fill your whole system with water until it reaches that same pressure.
The way to tell what pressure you want to see showing on your pressure gauge is to guestimate the difference in elevation between the highest point in the heating system and the pressure gauge. Multiply that difference in elevation by 0.4333 psi per foot. (See PS below.) If the number you get is anything less than 12 psi, use 12 psi. You need that much pressure to prevent cavitation from occuring on the impeller(s) of your water circulating pump(s).
PS: The reason why the water gradient is 0.4333 is because water weighs 62.4 pounds per cubic foot. So the pressure at the bottom of a 1 cubic foot aquarium would be 62.4 pounds divided by 144 square inches, or 0.43333 pounds per square inch. And, for every foot in height you add to that aquarium, the weight of the water increases by 62.4 pounds, but the area of the bottom remains the same, so the pressure on the bottom will increase by another 0.4333 psi.