First, ensure you're using the correct 15 PSI pressure regulator (Presto part 9978). The stove has reached pressure when the rocker moves rhythmically. If it's just hissing, pressure hasn't built up enough yet.
Check these things in order: 1) Make sure the sealing ring is clean and properly installed in the lid. 2) Verify the over-pressure plug is seated correctly. 3) Clean the vent pipe with a small brush to remove any food debris blocking steam flow.