Disklavier

Thank you Bill, your answer was most helpful. The metal flag in the 
bottom of the problem key turned out to be on its place, but ... I 
noticed that a small piece of paper had fallen into the slot where the 
metal flag fits in, and that snip blocked the electric eye. As a result, 
the release of the key was not recorded.
By the way, I was surprised about the amount of dirt I found under the 
keys. Some cleaning there from time to time can prevent things like this.
Thanks again for your response,
greetings,
Leon (Netherlands)