The shingles were not properly installed years ago and the roof leaked

This question is submitted by Rosalie Y. out of Massachusetts

I live in the attic of the condo. The roof leaked into my condo. The roofers uncovered the shingles around the skylights. My actual skylights don't leak. I have 5 in my unit. It seems when the second set of shingles was installed 20+ years ago, they were not properly installed around the skylights (no flashing). Hence the leak: 2 out of 3 owners agreed this was a common expense. One refuses to pay. Is this a common expense?

Thoughts?