I would recommend createing a ticket with the support desk with more information on what process you used to export / import the certificate. They should then be able to advise further on this issue.
I would also recommend looking through the Email certificate FAQ on our knowledgebase to see if there are any helpfull tips in there that could help with this problem. This can be found on the below URL
https://support.comodo.com/index.php?_m=knowledgebase&_a=view&parentcategoryid=32&pcid=0&nav=0