Hi Carey K - It's ok if your heels don't make it to the floor. As long as you are exercising the correct alignment when doing the pose (lengthening spine, sit bones up towards the ceiling, engaging "action" of heels pressing down etc.), you are still benefiting from the pose. It may just be genetics.

On the other hand, if you just have tight hamstrings, then try to place a rolled-up blanket under your feet. The balls of your feet will be on the mat, while the arches and heels are supported by the blanket. This will allow you to do the pose with straight legs, as the blanket brings the earth closer to you. This support will also prevent you from overstretching the hamstrings or straining the back of your knees. And having this padding to push down into, will slowly and safely help stretch your calves and hamstrings over time.

It may also be helpful to do downward dog and lunges against the wall before you start your practice. This will help to warm-up the hamstrings.

Hope you'll find the above helpful. Enjoy your yoga journey :-)