I don't know off hand why it wouldn't work, but make sure you have selected the right clip adjustment option:
Loop allows to loop the animation, but you cannot change the length.
Shrink/Stretch changes the length of the clip but all the animation is still there, thus changing the timing. For example, if the clip is half the length, it will be twice as fast.
Shorten/Lengthen changes the length of the clip and in the case of shortening trims the animation that doesn't fit. If the clip is made longer then you will have a part at the end with only a pose.
Another way to shorten the clip (which is destructive) is to place a break point and delete the second half of the clip.