I am new to vaping & have used prefilled carts and have also been filling blank ones. My prefilled seemed to last longer than the ones I have filled. My question is that I have put about 15 drops in a new blank cart and have experienced a few times that it doesn't seem like anything is in there after I have left them to soak over night. I do put the plastic caps back on and the manufacturer suggests letting them sit for 20 mins...so what am I doing wrong?