Take the barrels off and putting the receiver close to your ear, push the opening lever all the way to the right. You should hear a distinct click. I looked at a Parker the other day and the guy told me that you had to push the lever back every time to close it. It would open before taking the lever all the way to the right, and when one did, it clicked and locked in place just like it was supposed to. He said he had several from the same guy to sell that did that and he thought they had been modified. They had not.

You will find Parkers that will open without going over to engage the trip/spring/pin.

If it tries to hang and lets go or if the sound is dull, it will need to be cleaned or parts repaired.

Complete disassembly, cleaning and new lubrication tends to solve a multitude of annoyances. Many Parkers have not had that "luxury" since leaving the factory. A testament to the maker but a good cleaning once a century is not an excessive demand. Cheers. Jack.