So in the 3 flavor mix you used 15 mls of flavoring total, 15% of 100 mls? You should be tasting something really, however some commercial liquids do use a pretty high % of flavoring and plenty of sweetener, and you may be used to that.
Also a possibility that the flavors are working against each other a little somehow, it can be complicated with flavorings.
I honestly don't have any better advice. Give it a few more days. Try a new mix with maybe just over the recommended % for single flavor mixes, but in a multiple flavor mix make one the main and support with others...
Don't
throw out the stuff you have now though, give it a little more time.