There are three steps to the figure 8 join.
Insert the needle into the bottom of one piece either in the cast on edge or above the cast on edge from back to front.
Insert the needle into the bottom of the opposite piece from back to front. If you went above the cast on in step 1, go above the cast on in step 2. If you went into the cast on in step 1 go into the cast on for step 2.
Insert the needle into the exact same spot you inserted the needle in step 1 from back to front. If you fail to work the last step, you do not create a figue 8.
Remember, as experienced knitters, it's often easy to "predict" or expect to see directions or things in patterns. And sometimes we read the directions, but our hands do something else, because subconsciously it might seem more logical or it's what we think should happen.
This is what I think happens to many people when they try to work this join.
Common mistakes we see:
Step one is correct, but step two the yarn is worked front to back rather than back to front. This makes a figure 0 and does not securely join the pieces.
Step 1 and 2 are worked correctly, but in step 3 the needle is either brought up through the intial insertion poin and then under the first seaming bar. Or, the needle is brought under the first seaming bar on the original swatch.
If you do this, you will seam the cast on row of the second swatch to the first row on the first swatch. They will be misaligned. The bottom will not lie straight.
It gets even more disasterous for seed stitch, because the rows are now off by one and if you planned to carry the pattern over the seam, you won't, unless you make a fudge somewhere by working two bars from one swatch.
Here is a video.