Faye,
I think what you are saying is you are trying to sew the elastic to the inside of the completed waistband. I don't know if the completed waistband is already elastic or not, but either way, no elastic is going to be able to contract with three or more thicknesses of fabric stitched to it. my prefered way to do what you want to do is take an inch pleat at the side of the waistband on each side. then stitch the pleat down so it isn't uncomfortable. in the waist.
if that doesn't suit, you will probably need to remove the waistband, remove whatever stiffener (iron-on can be removed by heating with an iron) ,or elastic is inside so the waistband is pliable and add in your own elastic. if it already has elastic cut it shorter, if not, then measure the waist, then measure the front waistband and subtract from the full waist measurement.
cut the elastic to the remaining measurement, then sew it into the inside of the waistband and close up the waistband.
Hope this helps, Kitty