Who is the manufacturer of the flavours? It sounds like a cheap make if you getting no flavour at 20%
Also what type of recipes are you doing?
For example Capella & Perfumers Apprentice are usually good at 20% or there abouts.
I had some CCW flavours before and even at 30% they were very weak so i just binned them, its a false economy buying cheaper flavours imo, as if you need to use more, then the cost saving is negligible and tbh i would be uncomfortable using more than 30% flavour.
Fruit flavours require little steeping, 24hrs is ok where as dessert flavours eg. custards take at least 4 weeks imo.