My personal suggestion would be to add commentary in your videos, shorter videos would be nice as well. Certain games might need longer video length, but most of the time you can cut out boring parts and leave in the ones you find okay.
Thumbnails is nearly a must, of course it might take some time but it's definitely worth it in the long run.
Try playing different sort of game genre's to add to the content pile, people search for various games and will have an easier time finding you, if you got more content of different genre's.
But if you wish to just have a few type of games, you might want to be patient on the growth side for a while. Especially if you got competitors that play the same games. Don't let this drag you down, remember it's a sea of channels out there and you might be lucky to get on the hook a day!