It may be necessary to change some data in the database backup before importing it in the new site. Some plugins store "root relative" urls (I don't know how to call them) in the database, and those might not work on the new server (it happened to me).
For example, your site's root directory might be something like '/home/mhd/domainname/htdocs'
while on the new server it might be different: '/sites/domainname/htdocs'
In that case, you'd have to replace any '/home/mhd/domainname/htdocs' stored in the database with '/sites/domainname/htdocs'.