The best way to answer this question is to look at your landing page.

However, I will try to list few things that MIGHT cause this:

1. NO VALUE:
The user may perceive that there is nothing much gained in subscribing to your emails.

2. NOT ENOUGH TRAFFIC:
You may not be getting enough traffic to get email subscribers. Point is, not 100% of the traffic subscribes to email. It may be, say 5%. Now, if are getting just 20-30 users, it may not be enough to get enough subscribers.

3. WRONG LOCATION:
The place where you are asking for emails, may not the right location.

4. WRONG AUDIENCE:
It may also happen, that you might be targetting/attracting the wrong audience in the first place, who don't want to share their emails.

Best thing to do would be to give something in exchange of emails, like free e-book, whitepaper, etc. which would be useful to the user. Also, create multiple landing pages and do A/B testing.

This will increase your chances of getting more emails.

Hope this helps.