-Usually family and friends who are your subs check things out regularly.
-Also, if you want active subs I think you should communicate with the e.g if they post a comment on ur vids, you have to reply back to show them that you care about them and they have someone that gives them attention.
-try maybe every month inbox them one video to remind them or keep them updated ( but it can get annoying and lead to decreased subs)
-try to ask questions in your vids to make it more exciting and feature comments, everyone want someone to feature them. For example ask a random question like "who came first, the egg or the chicken" and you will have so much comments in reply because people want to find a place to express their comments so give them that chance. I like to call this the teleported effect on youtube.
-try to post at least one video a week. Sometimes subscribers forget that they had subbed you, therefore you have to remind them that you are still there
I hope this helped some.