can't gaurantee that i have found the answer but i was having the same problem and eventually convinced myself that the servos were faulty.

I removed the servos and set them up out of the plane so that i could measure the deflection and sure enough on full deflection (as needed on the funtana) there was a considerable difference between each servo (I am using jr 8425 servos not cheap).

I emailed my distributor who would gladly have looked into the problem but during our email correspondence he informed me that jr servos work like this. At 100% ATV the output shaft swings thro 60 deg and at 150% ATV the output shaft swings thro 90%. When i looked at my servos they were swinging through more than 100 deg (with 150% ATV) and that was my problem. I had installed one servo set it up for max ATV the other servo was set up exactly the same, one servo swings thro 100deg one servo swings thro 105 deg answer lower the atv to achieve the 90deg (or what each servo will swing thro) in my case this is 120%ATV then both servos will swing to the max they are designed to. Of course you may have to lengthen the servo horn. Hope this solves it for you as well