I think converting viewers into subscribers also depends a lot on whether your content is reaching the right audience.
For example, younger viewers, let's say under 25, often don't subscribe to channels even if they watch the content regularly. They just keep coming back through recommendations or their homepage. On the other hand, slightly older viewers seem more willing to hit the subscribe button once they find a channel they genuinely enjoy. At least, that's how I see it.
As for your gameplay channel, there's a good chance that most of your audience is on the younger side. And if you want younger viewers to subscribe, I think the content has to give them a really strong reason to do it. That could mean creating long-running series where people want to follow the story chapter by chapter, making genuinely funny and entertaining content with a lot of personality, or simply being exceptionally skilled at the games you play.