moonflower seeds collected last fall produced a 2' plant wit

Last year I collected seeds from several moonflower pods. I allowed them to dry on the vine and stored them in the house in paper enevelopes. I wasn't sure if the seeds needed to be winterized so I left some outside in a large planter as well. I am in zone five. None of the seeds stored inside germinated. The seeds left outside all winter did, however, these plants only grew to about 2' tall. The leaves were large lush and heart shaped. The stem was sturdy and the plant overall appeared healthy. It is now Sept and it never climbed or developed a single flower. I gave two friends a plant and they reported the same complaint. Fortunately i bought another from a garden center so i was still able to have a lush flowering vine again this year...but i don't plan on collecting seeds again. What went wrong?

Short season?