Not all heads are perfectly round and symmetrical, on the contrary, practically all of us have a particular skull shape. Therefore, there is no perfect measure. A wig can settle better than another depending on many parameters. So, generally, we should not think that the wigs will fit like a "glove" (in fact the gloves almost never fit perfectly either). It's about making the wig comfortable to wear. According to the shape of our head, we may feel that the wig squeezes us more on one side than on the other. That is normal.
If the wig does not sit perfectly on the forehead or sides, it is normal because of what was said above about the peculiarity of each head shape. That is not a defect of the wig. In fact, it is possible that even some wigs from the same manufacturer fit perfectly and others do not. When this happens, we must resort to the use of adhesives or double-sided tapes and place them in areas that do not sit well.
As we have said, it is better than "on" and not "missing", because a large wig can be adjusted but a wig that does not fit on the head has a bad solution.