About the Low Double Opt-in Rate

I have a website in a health related niche that I have a squeeze page to invite visitors to double opt-in my newsletter in order to get a free ebook. I used Google AdWords and Facebook to promote my page and the response is good. Out of every 10 targeted visitors, about 1 to 2 will opt-in my newsletter. I use a self-hosted mailing script on my shard hosting server to handle this.

However, problem comes as I use a double opt-in system. For every visitor who have put down his/her email address, my email server will send out a confirmation email. My statistics show that for every 3 visitors who supplied their contacts, only two of them finally confirmed by the link in my confirmation email.

That means, I lost 1/3 of the visitors because they never clicked the confirmation link.

I think there are possible reasons for that:

1) They never received my confirmation email due to delivery problems of email.

2) They did receive but somehow they never click the link (due to ignorance or some other reasons). I have already put down instructions in my webpage to advise people to carefully check this email and somehow if they did not receive the confirmation email, they have to email me.

I am now thinking if most of the loss are because of reason 1. If this is the case, then I might need to find out the cause and hence the solution

I have the following questions in mind now:

1) Is it because my confirmation email has been filtered by the spam system of the visitors?

2) Is it because of some other reasons I have not though of?

3) If I use email newsletter service like Awber.com, would it help to solve the problem(s)?

4) Any people have similar problems? Or if 1/3 is a normal loss-rate for a double opt-in system?

Any experienced people can throw me some light on this?

Thanks

Damen