@Anonymous User Welcome to the forum. The previous seasons were available as boxsets and left on 26th as that was the expiry date of them. Season 6 is the latest season so that is still available as catch-up content.
NOW TV doesn't have the rights to keep shows on the service forever, so all boxsets have an expiry date. When shows have 30 days or less left, you will see a countdown reminder message appear next to the episodes on the website and most apps to warn you. It's worth looking out for these to help you plan your viewing.
The boxsets should appear again sometime, but NOW TV don't have any date for Elementary yet.
For more info on how content works on NOW TV see the following help page
http://help.nowtv.com/article/Why-can-t-I-find-the-show-I-m-looking-for
