When working with stretchy fabrics, frequently one side will stretch a little bit more or less than the other. If left unchecked, you end up with a mismatch in length by the end of the seam. Or even worse, in a bad attempt to fix the problem, the seamstress/seamster ends up with tucks and folds in the most inopportune places.
A lot of this can be avoided with careful pinning. But if you notice that maybe the top fabric is stretching a bit long and the bottom is a smidge too short, before it gets out of hand, start stretching the bottom fabric as much as possible while gently pushing and easing the top fabric under the presser foot. Careful: don’t push too hard, or you will end up with those tucks after all. Reverse directions as needed (i.e., longer bottom and shorter top fabric call for pushing and easing the bottom while stretching the top).