Are you offering any sort of incentive for joining, like a lead magnet? If not, I strongly recommend you consider setting something like that up. People are far more likely to join a list when they're getting something out of it, compared to joining a list for the sake of joining.

I just recently shared an article on this exact problem if you want to check it out:

https://zylerkade.com/this-5-minute-lead-magnet-strategy-has-made-over-100-million/