Yep, that happens to me too. Some folks can go for months on the same flavor and be as happy with it as the day they first tried it. I'm not one of those people.
Like mentioned above by a fine vet, vary them up. I have at least 3 different flavors each day and seldom have the the exact same mix the day after. If I don't then I get to the point I cannot taste a flavor that I really like and I have to stay off that flavor for months before I'll be able to taste it again.
This is the second reason I mix my own. (The first reason has nothing to do with this topic.) I have a number of small bottles of mixes that are "in development". They are not perfect yet, but close enough to be a good vape. So my latest tweaks are carried around in my pockets for taste-testing and to change up my flavors. (I usually have a number of attys on my person too as some flavors just do not mix and blowing an atty out is not polite in mixed company.)
And, some juice flavors do lose their level of taste over time. Some get stronger too and some don't change at all. (This is all according to my experience.) The ones I know to lose over time are kept in my 'fridge and I only carry a 3ml bottle from those on me. This means I carry a bunch of little bottles and wash and dry a number of little bottles with my jeans. So may not want to follow me down this path. Find what works for you. It's worth the effort.