With all due respect to other opinions, most modern piezos don't "quack" like they used to, especially if you know how to play, pre-amp and EQ them. I think Fishman does a great job with them, and whatever ones are in the DTAR system work great too! And I have never really found a soundhole mag pickup that I liked tone-wise or string-to-string volume wise. And I don't like putting electric strings on an acoustic. ( However, I did just that with an old J160E I had back in the day!)
So, on to your issue! You can simply replace your UST with a number of good options, if it comes to that.
But, if you're trying to salvage what you have, there are a couple of things you can try first!
As mentioned before, make sure the bottom of the saddle is flat and true. Make sure the slot on the guitar is clean and flat and true.
And make sure the saddle fits snugly in the slot. VERY snugly.
I've had a few string-to-string volume issues with piezos on a few different guitars. Usually one of the E strings is too soft compared to the other 5.
After making sure the saddle is flat and the slot is clean, reinstall the piezo, plug the guitar into an amp, turn it up just enough so you can hear what you are doing, and do the tap test lightly and carefully along the entire length of the piezo strip to make sure it's working correctly. Then install the saddle and string it up, but not to pitch...just enough to hold pressure on the saddle. Then do the tap test again all along the length of the saddle, listening carefully for volume all along the length. If that fixed it, then you're good to go! Tune to pitch and check it out again.
If the volume on one of the strings changes again, loosen the strings a bit...try to rock the saddle back and forth (towards the neck and back towards the pins) in the slot while you're tapping along the length of the saddle. If you can rock it, even slightly, you'll probably hear the volume change as you are tapping. Hold it to where the volume is even and note what you're doing. That's where you'll need some sort of shim on the side of the saddle. Sometimes it's such a small gap, you can only use a small slice of scotch tape or two as a shim. (That's what I had to do on my "new" Gibson SJ200! Works perfectly now!)
Sometimes the sides of the slot or the sides of the saddle will wear ever-so-slightly with playing. Sometimes it's just not quite right even fresh from the factory. Sometimes the pressure of the string at-pitch will accidently hold the saddle in the right position, and then when you change strings or tunings, you all of a sudden develop this issue. (That's how some of them get thru QC) On an acoustic without a piezo, you won't even notice it. If you have a K&K, or a system like Rayk's, you won't ever hear it. But the under-saddle piezo is very sensitive and it needs to be near perfect in the slot and have a near perfect fitting saddle with near perfect pressure along its entire length. It's kinda finicky, but when it's set up correctly, it works! And the more modern Fishman USTs work great, especially when playing at band volume and you don't want feedback!
I hope I described the process adequately. And I hope it sorts out your string-to-string volume problem.