E liquid losing a LOT of nicotine strength over time?

[newvaper92]

I usually use 3-6 mg liquids in my Subtank Nano (powered by an Istick 30) but have a 15ml of 18mg (from Mt. Baker vapor) that I occasionally drip directly onto the coil for a quick nicotine buzz. The bottle was from march, and I'm just getting to the end of it. I recently got another bottle of 18mg from my local vape shop, for the same purpose. It was in their (rather extensive) 50% off bin. Not going to name this one, but I couldn't find a date on it. I'm assuming it's old (older anyway) because I get no buzz or anything even chain vaping it. The flavor is great, but it seems to have little nicotine in it. It is really smooth, which 18mg rarely is. It was still factory sealed and everything. I thought for a couple days I'd built up a tolerance to the 18mg, but no, I tried the other one and it knocked my socks off after 3 hits. 18mg is 18mg, right? Can strength vary that much between brands? Or is it really old and lost strength? I also bought a bottle of much lower nicotine from the same 50% off bin, different brand though, and while I think the nicotine content is "as advertised", it is a max vg liquid, and burns my throat so bad I don't want to vape it. Flavor is ok. Not great, but there. Never had a max vg do that before. Just a bit confused is all. In the meantime, I'm vaping the "18" mg in my tank as I would a 3-6. No difference as far as I can tell.