When you use hot glue to glue fabric together the hot glue should dry near instantly after you put the hot glue down on the fabric and press it together.
It can take a few minutes for the glue to fully set and the fabric hold together after you apply the hot glue.
If you're gluing fabric together use a putty knife or something else to help press the fabric together before the hot glue does cool down.
That will ensure the fabric stays together with the hot glue.
Apply a generous amount of the hot glue to make sure the fabric stays together.