I've been stretching my legs recently as they were feeling a bit tight. I'm doing a series of different stretches but I've been concentrating mainly on my hamstrings. I can now touch my toes for 60 seconds (longer if I wanted, yay!) however I've noticed I'm struggling with one stretch in particular...

When I stretch my hamstring by [laying on my back and using a towel](http://www.somastruct.com/wp-content/uploads/2012/11/TowelStretch.jpg) to pull a leg towards me I noticed it still hurts and I cant pull my leg straight yet (my left leg can go further than my right). However if I use a [door frame to stretch my hamstring](http://flexpt-2734.kxcdn.com/wp-content/uploads/2015/07/Wall-HS-finish.jpg) I can get my legs completely straight and without anywhere near as much strain on my hamstring.

I'm trying to understand why by using the towel I cant get my legs anywhere near as straight as I can using a door. Do I need to concentrate on stretching some other muscles with this?

My end goal is to be able to lift my legs straight up in the air without having to use a towel or wall. I'm 24 and female. Any advice you guys have would be great. Thanks.