To prevent "hourglassing," use the "Weaver's Four:"
At the end of the row, pull the outer-most warp string out to make sure it
is not tight.
Make a "bubble" with the weft by allowing it to mound in the middle of the
Pack it into the weaving with a comb or a fork.
Strum the warp strings with the fingers of both hands from the middle of
the warp to the outer edges to keep the tension loose.
If it is any consolation, it is a common problem even for experienced weavers.