String frequency is too high in the lower and upper middle part of the frame, if not hit in the sweet spot, more in the center of where you like to hit. The strings are shorter there, and closer to the edges of the frame, causing higher frequency on rebound and no control. (Too low towards the bottom or too high towards the top edge of the racquet.) Jammed engines: lower and upper body is different in the way we gain coil and uncoil. Lower body depends on choppy fast martial arts like foot and leg work, while upper body depends on loose, relaxed, speed.
Most ues are made due to either coil errrors, or contact point errors. High balls have to be hit way out in front towards the net. Low balls are best hit more close to your body, due to your length of arm at locked wrist contacts.