you may have a point with a few things. but you also have to remember that when yoda had trouble lifting the xwing, he was also very old and weakened. and had been out of touch with The Force for some time so as to not be felt through it by vader. and when you consider that there are only a fair few that have the ability to use and focus The Force, that greatly decreases the extent to which an individual can accomplish with The Force alone.