If one of your rope ends is started in the wrong direction, then the two knots never perfectly fit together. It would look like this below.
The following steps will assist you in tying your double fisherman's knot correctly. The key is to use the contour of your hand to direct the knot. Start by pointing your thumb to the rope that will make the fisherman's knot.
The other end is held with the hand and will be tied in a later step. Next take the other end and wrap it around the thumb twice.
When the thumb pulls out, there will be a clear channel through the two loops. Insert the end of the rope to make an interim knot. This happens to be the anchor hitch, and it makes a good transition.
Roll the anchor hitch into a fisherman's knot by setting the loop closest to the end over the other.
Now dress it tightly to form the first fisherman's knot.
Rotate the knot over, moving the fisherman's knot from the left to the right.
Using the same hand as before, tie the second fisherman's knot.
When you finish, you'll have a double fisherman's knot that nicely fits together.
Here's an example of a loop.
Rotated around, the opposite side is shown below.
Here's the video link: