The September 11, 2001 attack on the US remains the only time NATOs article 5 has been invoked. But during the existence of article 5 has any NATO country suffered a significant qualifying attack (see article 6) without it being invoked?

Politics can certainly muddy the water here and fiddle with the meaning of what constitutes an attack so I'll ask that two categories be considered when defining "an attack":

1. The actual language in the treaty
2. The attacked NATO country requested that article 5 be invoked

This means answers may include both requests for invocation that failed and qualifying attacks that didn't produce a request.

But essentially what I want to know is, after all this time, what explains article 5 only being invoked once: a failure to honor the treaty or 73 years of peace?