Let me first say I don't know a darn thing about the flavors you are using specifically.
Some flavors need time to develop, Double Chocolate (BV) is one I use that needs time to come through to full strength. The same flavor by itself tastes pretty much like cocoa and not exactly like chocolate at first, with age it mellows but retains the taste quality. Adding a bit of butter and cream flavor transforms the flavor into proper chocolate.
Many times we taste fruit and think "yum, orange" but we don't contemplate exactly what's going on in our mouth. That contemplation is what helps you locate missing aspects of a recipe.
So let's say you grab some fruit, sit down and taste one; then write down a description of the flavor as best you can. Then do the same with your juice version of that fruit. What's different? More or less this or that at XX% flavoring level.
Don't be afraid to go super low on percentages either, some flavors are clearly correct at less than 1% and come off as a chemical or perfume at 3%.
So a single flavor strawberry juice might need more than just strawberry flavor, sweet, bitter, dry, wet, thick, tart, dull, harsh, hot, cool, and so on. Most fruits are 80% or more water then lots of fructose a bit of acidity then an almost microscopic bit of the things that individualize flavor.
Try marshmallow, sweet cream, or maybe cotton candy, with the individual flavors of fruit to see if you can bring out a more true berry flavor. Then fine tune for missing notes once you find you are as close to the proper sweetness as you can get. I wouldn't be surprised if just strawberry needed 3 or more flavor components by itself.
Get single flavor aspects nailed then move on to the next aspect, blueberry for example. When you have the individual components nailed experiment with combining two components. Flavors A and B, B and C, then A and C to see how each flavor interacts.
Your total flavoring percentage will be whatever it is, don't worry about it, just make sure when you test the flavors you are letting them settle (steep) to their final flavor. Otherwise you'll be chasing something that's always running away. Flavor changes a ton over time.
I don't know if any of that helps, I hope so.
Maurice