Any critique help woulld be loved!

Hey everyone! I have been loving this website since I discovered it. I use it every day for 2-3 hours depending on what else is going on that day. But lately I have been feeling stagnant. As if My art has hardly been improving! Any critique at all with what I am doing wrong would be greatly appreciated!

So in order to do so I will be posting many of My sketches from when I started using the site to now!

http://imgur.com/a/KgmAK
The top two were My first 30 minute sketch's using the site. Going down the list are leading down to My newest ones.

So guys, any ideas?