Possibly you need to adjust your regulator but don't try that until you try this.
With the new style OPD tanks the male fitting must screw in deep enough to depress the valve fully to get max gas flow - in the female tank fitting there is a small brass ring that can back off and not allow the valve to fully open.
tighten this brass ring and you troubles should go away, or take the tank to a good LP service center and they should have the correct wrench to fix it