You didn't provide a specific recipe name, so I am guessing here, but is this the recipe you used?
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If so; I note that you did not follow the recipe. You included half again as much strawberry, and tripled the watermelon. So, it is not surprising that it did not come out like you expected. That would be like taking a pancake recipe that calls for flour, baking soda, milk, and eggs, and deciding to add a random "extra" amount of flour and baking soda, and still expecting it to taste like pancakes.
One of the things that is nice about DIYing your own e-liquid; is that you
can make juice any way you like. You can increase flavors, decrease flavors, and even substitute flavors and brands... but you
do need to have an understanding of the flavors you are working with.
Because, where my pancake analogy falls on its face is: flour is flour, and baking soda is baking soda, and any brand will do; that doesn't translate well to DIY. If I try using FlavourArt or Inawera Strawberry, in the same percentages, as I use The Flavor Apprentice Strawberry, I would have something completely unvapeable. This is true too, of arbitrarily "adding more" of any flavors I am unfamiliar with.
Get to know your flavors. Work with them. Learn what they do, and don't, contribute. Doing small (3-5mL) test batches of single flavors, at various percentages, is a good way to do so... and learn what percentages they work best
for you.
As others have suggested; this can be due to using too much flavor. For some vapers, some flavors
can actually start to fade (or disappear) when to much is used. It seems to overwhelm the taste buds and cause them to shut down for a bit. This could be an explanation for why it seems to linger for a bit, and carry over to your store-bought juice.
Or, it could be a poor quality batch of one of your bases. I would do some googling of your specific brand(s); and see what others are saying about it.
Don't rely on the vendor's own posted reviews, as some unscrupulously omit less complimentary opinions.
Also, it is fine to vape PG or VG, by themselves. It isn't very exciting, but it will show you quite quickly, if either of them is bringing something unwanted to the party. Give yourself a clean coil and wick; and then add a few drops of PG to the atty; take a few puffs; repeat until you are satisfied that you detect nothing "off." Then; do the same thing for your VG.
You can't/shouldn't do the same thing for your nic base... so for that, you will need to mix up a small amount of unflavored e-liquid.
After you have tested your PG and VG (each separately); mix up a small sample of PG, VG, and nicotine base in your preferred ratio (65P/35V 4mg/mL nic?) and then vape that. (As a side-note: there are those of us who actually like unflavored as all, or part of our everyday vaping routine. You may find you like this and get an easy, and cheap, "winner" out of this experiment.
)
If those all work fine for you; then you have ruled out everything but your flavor concentrates!
I had two thoughts as I read this statement.
First; about choosing particular recipes. Besides looking for recipes that I have the ingredients required, and that it is a flavor profile that sounds good to me; I also try to choose those that have a fair amount of positive feedback. I notice the strawberry/watermelon recipe had NO feedback, while the Snake Oil had a significant 32. Depending on what you are looking for, you may not always get to be so choosy, but it is something to think about when searching.
Also, while I am on the subject of scouting recipes, I always make a note of who the contributing author is, and what website I found it on. That way, if it is a winner, I can easily seek out other recipes by the same author.
The second thought was; I am still really struggling with your description of your efforts as "oily." To
me, oily is less about taste, and more of a "feel" thing. I am wondering if it might be more of an "artificial, chemical-like" off taste that some people can detect with certain flavors (esp. when used at higher concentrations).
I don't personally. As I understand from what I have read: the two main reasons some choose to dilute is either to thin it for atomizers that don't wick high VG e-liquids well enough (primarily tanks); and some think it creates a "moister" vape (my own brief experiment did not result in any benefit).
The other thing to keep in mind is, if you are using true USP grade VG, there may already be water in it. Look for a purity percentage (e.g. 95%, 97%, 99%, etc.) and know that the balance is distilled water; so be sure to include that in your total dilution calculations.
