Ithaca NID problem

I took my old 12 ga Ithaca NID on the South Dakota pheasant trip this year. The first day everything worked fine, no problems. The second day the second time I fired it one trigger worked but the second one would not function. I noticed the action had opened an tiny bit. Thinking I had moved the lever accidently with my glove hand I dismissed it as my fault. The next shot it happened again and every shot after that it until I decides to stop shooting until I could retrive my back up shotgun. Both days I was using 2 3/4" Fiocchi golden pheasant #5 shot. The shotgun was clean and properly lubricated and seems to lock up tight. But something is wrong. Does anyone know what the problem could be and where I could get it fixed? I live in central Florida. Thanks for any help or
suggestions.

EDIT: Refreshing in case some one missed it.