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I have seen this issue before. Number one, ensure you've not installed a gas line with a "Safety flow" nipple. They just don't work properly. If you find this, remove it and put on a new nipple.
Number two: The rating label for gas pressure tested with a manometer should be around 11"WC on a propane stove. If those two things test ok, you're going to need a new pressure regulator.
I can't stress enough, if the installer of the gas line used one of those "Safety flow" nipples on the gas line REMOVE IT!!!!