If this is a milestone you hit more than 30 days ago, our app only picks up milestones you've hit in the last 30 days. In the future we do plan to support milestones older than 30 days, we just haven't added that functionality yet. So you won't be able to get this milestone.
If you are only missing subscriber milestones, then the issue is likely that you've chosen to hide your subscriber count on YouTube.com. If you change your YouTube settings to hide your subscriber count, we don't have a way to see your subscriber count so we can't give you milestones. To start getting subscriber milestones again, all you'd need to do is go into your YouTube settings and make your subscriber count visible.
If you are a newer channel, we do not start tracking milestones until you've reached 1+ upload, 10+ subscribers, and 100+ views. Starting 24 hours after you've met all 3 criteria you will start getting milestones.
Finally, it's important to note that when you get milestones change over time. For example, if you have less than 100 subscribers you get a milestone for every 5 subscribers, but when you have 101 - 250 subscribers you receive a milestone for every 10 subscribers. As you grow, it doesn't make sense to give you milestones at the same rate because you start growing much more quickly.