You bring up a good point about replacing every six months. Ours are definately older than that. Perhaps the durability of the dispenser erodes with age. We are trying something new, though. Are you familiar with the Glade plug ins? They disburse scent much like the feliway dispenses pheremones. We bought a Glade plug in, dumped the scent, and replaced it with Feliway. We've used the scent dispensers for a long time and never had a "burning" issue. They are much cheaper than the Feliway; we paid $4.34 for the scent dispenser.

It must be working, because my problem kitty is sitting right next to me and she very rarely does that! I'll keep you posted on any issues; I would hate to stop using Feliway due to safety issues.