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!

https://www.instagram.com/p/BEkocbxJKAf/ done in March this year.

http://imgur.com/z3EYrw7 Another one done a few days later.

https://www.instagram.com/p/BFpLcpDJKCf/ This was done last month.

http://imgur.com/UScslbg Another one done last month.

http://imgur.com/7hps6Ue Done a few days ago.

So guys, any ideas?