I'm using the time stretch setting to slow down a shot of tree branches passing above a car on a beautiful fall day. This is a GOPRO shot, by the way. I applied the time stretch setting and clicked on frame blending - but the end result was not useable. Various parts of the shot jumped and jittered attempting to blend the frames together - frame blend just wouldn't work.
Did you checked on frame blending in the composition or in the footage itself? Both need to be on for you to preview it. If both blending options don't solve your problem, time remmaping won't. You may resort to use a third part plugin like twixtor.