Can't really figure out the problem from this picture, but you might want to check if the transformation point of your symbols is set correctly (the point about which the symbol rotates).
OK, it's pretty easy to get this effect (at least in my Flash CS6 and my computer, it does happen all the time I do the following):
1 - Create new document with large size (let's say HD - 1920X1080)
2 - Create a line that covers about 2/3 of the area.
3 - Convert the line to a Symbol.
4 - Move the line's anchor point to its basis, so its rotating as if it were an arm.
5 - Create another keyframe at frame 30 and rotate the symbol about 1 degree.
Now turn onion skin on and check the inbetweens Flash creates. They are not equally spaced (smooth movement).
This problem seems to happen with both classic and motion tween, trying other methods, the only tool that gave me the desired results were bones.
hey kglad... Zoom in some more and probably you'll see the problem.
You have 30 frames, you were supposed to have 27 inbetweens the frame 1 and 30, but instead you have only 3!
If you play your animation, you'll see the line jumping from one position to another instead of just going smoothly from its position in frame 1 to 30.