This is a not uncommon issue with Yamahas, David. Most likely, that reed’s gap needs to be adjusted, which is a simple procedure. Do a search in the Forums for “gap” or “gapping” and you will find all the information you’ll need on this.
Another, much less likely, explanation would be that the reed has failed and has a fracture that’s not visible.
You should also check to be sure the reed is not misaligned. Push it gently through its opening with a toothpick to see whether it catches on one side as it goes in and out.