No Shares After Switching from http to https
At first i am sorry to say but all shares from old posts are lost, except the ones which are stored in the MashShare cache.
When you switch your domain from non ssl http to ssl https than the shares for the old page are lost because the links are different for google. When the cache gets recreated Facebook returns the shares of the https link and not the ones of the http link.
You can check this by yourself with visiting the following link and changing the link to your site: http://graph.facebook.com/?id=http://yourwebsite.com
As a solution you could manual put the old share count into the custom fields of the mashshare plugin but keep in mind that they will not increase until the new share count for that link equals the share count you paste into the custom field. Read here how to do so: http://docs.mashshare.net/article/26-modify-post-share-count-manually
If the shares are still existing in the mashshare cache (they are when you never used the MashShare purge cache function)
it would be possible to write a extension for mashshare which collects this old shares and use it as basis for the new share count.
But this would be additional work of a few hours. It all depends on how important old share count data is for you.