HELP: My Lighters Won't Stay Lit

Vin, here's my take on your situation based on what I've learned of combustion reactions while studying Organic chemistry thus far.

Propane and butane both both use oxygen during combustion in order to produce energy. In your case propane is used in your heater whilst butane being used in your lighter's. Butane is composed of a 4 carbon chain whilst propane is composed of a 3 carbon chain. In order to have a balanced chemical reaction the higher carbon chain fuel will need more oxygen in order to fully combust. The higher carbon chain being butane. This part of my answer is fact.

I hypothesize that the propane is getting sufficient oxygen for combustion, at least for a time, whilst leaving the butane with insufficient oxygen, being a higher carbon chain, and therefore needing more oxygen than the propane to react. This is the best answer I have as to why your lighter's won't work in your garage while the door is shut and the heater is on. According to this hypothesis, if you had a lighter which used methane or ethane, it would likely work.

There is slightly more to it but that's about the gist. You really should leave the door open though because the propane in your heater will not fully combust with a limited oxygen supply and will therefore also produce carbon monoxide which will bond to your hemoglobin and deprive your cells of much needed oxygen. Since carbon monoxide is colorless and odorless it is undetectable to humans and can often be fatal before noticing symptom's of it's affect on the body.

In short, leave the door open. I like you and would prefer that you remain alive my friend. If you do fancy the door being closed then I implore you to get a carbon monoxide detector as was stated by other BOTL on this thread. I hope this was a helpful answer to your question of why your lighter behaves the way it does in your given situation.