On many atties you need to clear primer fluid first, which tastes like gaack. Blow through them from the connector end, tissue held over the mouth end. Then try a burn with just a little plain VG or PG, to clear any final residue.
You should be good to go. When testing a new flavor, typically around 4 drops onto the top of the atty, may go to 6. Repeat the plain PG or VG to clear it before trying another blend.
I use a pretty cheap, simple, generic atty for testing, the only thing I'm careful about is that its resistance is the same as my cartos. If I've done multiple flavors, at the end I'll rinse under cold water, then a bit of PG or VG and a short burn, then put it away until next time.