There are two possible reasons.
First, drafts (pages never published) in SLS1 are not converted over to SLS2.
Second, there's a known bug that will be fixed in our 3/17/18 release. Please submit a support ticket if this was the case for you - we do have a way to convert those pages for you.
In the first case, we recommend the following as a way copy over the content of the missing pages:
- Unpublish your SLS2 website if published
- Create a new page in SLS2 in Site Center
- Locate the original unpublished page in Webmaster Center (old website)
- Copy & paste content to the new SLS2 page
- Publish the new page
It will be easier to copy and paste content between the two systems if you have two browser windows or tabs open at the same time.