Have you checked inside the wall switch itself? We've had loose wires show up years later.
We replaced our living rm ceiling fixtures in July. Had replaced the factory ones in 2010 because they were overheating and caused a burn mark in the plastic ceiling. The factory sent us 2 identical brand and they were beginning to darken inside.
So I looked online and found Tweety's had the cheapest price for long 30W that would use the F15T8 bulbs. Went with Thin-light brand (51-8122) Made in USA, instead of factory installed Made in China. Because of the burn marks, we had to get the long Euro-style again.
Tweety's had fast service. Once again, PPL is $10 each higher.
http://tweetys.com/search.aspx?find=55-8122