You just run into a bad one. Get another name brand (Stant etc) & work the plunger several times to make sure it is free then suspend it in a pot of water & bring it to just below boiling & confirm that it opens. Best would be to use a candy thermometer to confirm it opens within ~5 deg of it's rated temp. A Moroso racing stat might be a better choice yet. No Robertshaw, when it came out it was touted as a trick piece (not sure what the trick was, maybe it opened further) but my parts man couldn't believe the # of comebacks they had with em