Gotcha. It could have been a lot of different things. Weather plays a huge part in candy making, dipping, etc. I'm not great on the science side of it so I'm sure Michael in FtW or Goodweed will be able to add to this too. I'm thinking maybe the humidity was too high, causing your pretzels and cookies to be soggy and the chocolate to not set up well. Also the oreos might have been stale to begin with. I know when they go stale they do get soft like you said. Also, try letting the melted chocolate cool a bit before using it. It's a fine line between keeping it from getting too cool so that it's still smooth enough to dip but not too hot.
-A balanced diet is a cookie in each hand