I do hope that someone may be able to help with this problem. I have kitchen under unit lighting comprising four linked fluorescent units. They are all the same make and use T8 triphosphor tubes. The array, controlled by a single on/off switch, is made up of 18”/15w to 36”/30W to 18”/15w to 36”/30W. Everything has been working fine for years until recently when the 18” tubes started to fail (to start). I have replaced them about four times now, but each time they last only about three weeks before failing. I have eliminated the starters as the problem. The 36” tubes are unaffected! Any ideas gratefully received.