Went through the same thing last night. All the JR 6xx radios appear to have this problem. Whats happening is the flap channel is getting mixed with the ailerons and flaps are inherrently all the way down or all the way up so it screws with the center position of the servo. Reset your model. Set to flapperon mode. Then set up a mix of channel 6 to channel 6 (flaps to flaps) and set the amount to -100%. This will allow the true servo centers to be proper. Then go into the travel adjustments for flaps and set appropriately for the amount of flaps (drooping ailerons) you want. Search rcuniverse for XP652 and flaperon and you should find my original post that I answered myself when I found a relating article. It has a link to the article that shows this method and another.