From what I can see, you play a lot of different games, but only have a few videos for each of them. You may be getting people subbing who like just one of those games and when they come back you are on another game. NOt that it is a big issue playing lots of games, but not everyone likes every game. It's hard to tell by just looking at your video list when you actually upload them, without going through every video and looking at the date. But it looks like you are uploading around 2 videos each month of the same game. You really need to upload more often for each game if you want to keep fans of certain games happy or they will lose interest.
Also as
@MemoGaming said, you probably gained quite a few of them from the giveaways, a lot of people will just sub in tyhe hope of more giveaways and not because of the content.
I don't know what your SEO is like but have a look at it and make sure it's perfect, or close to.
Get sharing on social media as much as you can without spamming, make sure people know you are there and uploading.
Also check the retention rate of every video and see how long people are watching and which ones they watch more and longer of.
Just a few opinions