Once the liquids are mixed, everything should be evenly distributed. Some flavorings tend to take longer (maybe days after first making eliquid) to evenly distribute / interact with the base liquid. But the nic won't 'settle' out.
I agree with the points that Mickey made, plus how much you vape within a certain time. With analogs when you finish one you're done; with vaping, you can 'vape too much' and eventually get increased TH, nic buzz, etc. If you are getting an increasing effect from the liquid, this could be the cause; not that the liquid is changing.
Hope this helps