Try to write some other pages and see if they index. That way you can determine if its a domain thing
my thoughts too... It's possible he's using a (expired) domain that was penalized before expiring, and now google refuses to give it love....
Why only some page are getting noindex and sometime they get indexed automatically.
well OP, only google knows this and the best we can do is guess
Anyway, if your client's domain is not penalized, from my experience the reasons for deindexation are usually one of these:
- glitches in the matrix (ie, google has bugs in the system, I think they've had 2 or 3 indexing issues in the last 12 months, I remember that 2 of them were confirmed via Twitter I believe)
- DMCA reports by competitors / someone else (I might be wrong about this, but I think google doesn't tell you about all DMCA requests made for your site, so it's possible that one of the pages gets a DMCA request and google simply takes the URL out of their index without letting you know)
- faulty / misconfigured plugins (sometimes, Wordpress plugins - I assume you're using Wordpress - noindex stuff on their own, or maybe you set the plugins accidentally to noindex important stuff, I've done this several times in the past, so I know that it's possible

)