More than likely the temperature wasn’t right when you microwaved it or hot water bathed it. Or some of it stretched and the rest cooled. It helps to do this in batches; split the curds into 4 parts and stretch one part at a time. The curds can even be refrigerated before stretching so there is no need to rush. Aside from using a thermometer, you can test for stretchiness by taking a small marble sized ball and kneading it, if it is rubbery, gets shiny and stretches, you’re on your way!