Just my opinion, but when I initially began vaping ejuice I could not feel the nicotine in my system, until it was too late & the headache set in. I did not realize how much of a learning curve this all took, plus I was craving cigarettes & that's where my mindset was.
My best advice to anyone is to take it slow & listen to your body. Now, each morning when I take my first few vapes I can feel the nicotine in my system...but it took a while. Ejuice nicotine is such a clean delivery system, its easy to think that its not working or that you need more, more, more.