So you've just sewn your knit neckline/neckband carefully and correctly and STILL end up with The Gapes, like this:
And then you end up with a big case of The Mehs when you had such high hopes for a cute top/dress using that pretty fabric you just bought (or sacrificed from the stash).
Well first, stop beating yourself up. It happens to all of us and doesn't make you a sewing failure. Sometimes bad things just happen to good fabric and/or techniques. Next, get yourself some elastic thread, like this:
And a big-eyed, blunt-end needlepoint needle, like this (the blunt end is important so you don't snag the fabric and make a difficult task of threading the elastic through):
Cut a LONG piece of the elastic thread, roughly 2-1/2 times the length of the neckline area you need to fix, stretch it a couple of times, and thread your needle with it. Then, push the needle under the stitching that's holding down the neckline/neckband, starting at one shoulder seam.
Make sure you get the needle up into the space above the stitches, like threading elastic through a casing (which is pretty much exactly what this will end up as). Work the needle/elastic through the whole length of the gaping area and leave a long tail on the end you started from (this is why you need to use a long length of elastic).
Push the needle out at the back of the other shoulder seam and into/through the serger stitches (or seam allowances if you haven't serged) of the shoulder seam, and ...
... tie a knot. Make sure your knot is behind the shoulder seam, since you've pressed those to the back (right?) and want any elastic tension to pull toward the back.
Next, go back to that long elastic tail you left at the beginning and pull the elastic taut enough to cause rippling but not enough to create actual gathers. The neckline isn't going to lay flat now, but that's OK because it *will* lay flat on you.
And tie it off at the starting shoulder seam, again behind the actual seam. (If you leave your needle threaded while you do this, getting through the loops of the knot is easier.) Finally, unthread the needle and trim your thread tails.
Now be happy again because your new top/dress with that fabric you can't believe you just "wasted" is now a keeper! Look Ma! No gapes and ...
... no gathering.
I won't tell anyone if you won't. ;-)
This little fix works for front necklines/neckbands too. Just run your elastic through the front instead of the back and make your knots in front of the shoulder seams. If you need to fix back AND front, do it in two steps, using two lengths of elastic (or 3, if you have a vee or wrap neck) so you can keep the "drawing up" effect of the elastic separate, which will work better than one long piece of elastic thread doing all the work in different locations.
You're welcome. :-)