Why dont' all DVD copies work?

I have never had any problems with Shrink but my problem is I copy DVD's and some play perfectley and some don't.
E.G. I made 3 copies of Shrek (for family)for 3 different players.Two play perfectly -1 becomes pixelated and looks like a wash board effect eventually jams and freezes the player.This happens at different points in the movie.
It also can happen at random on my expensive Panasonic machine yet when you play the same copy in a cheap player it MAY be OK.I have even burn't at 2X instead of 4X this does not seem to help.The length of movie also seems irrelevant so compression rate is not the problem?Have tried all brands of DVD blanks,no difference.There is no logic that I can see to this problem.A movie that has played OK one day and replayed
another day may jam usally at about half or more way through.Any one had similar unexplainable problems.??