In the seetings for IMAP accounts you can choose to download messages for offline use. There is a suboption to include attachments and images. If I check this suboption it will work fine and attachments are included. But after a while, the check makr in the box is disappeared and attachments and images are no longer included. Is this a bug or?
What version of eM Client and Windows are you using?
I have Windows 10 and eM Client 7.1.33101.0
I have not experienced this in that version of eM Client. I thought maybe you were on the beta version with all its bugs. 
Maybe close and uninstall eM Client. Then download the installation file again from http://www.emclient.com/dist/v7.1.33101/setup.msi and install it. You will not lose any data.
See if that makes any difference.
I forgot to mention I have the same problem on both my computers.
I have not seen this issue before. Maybe an eM Client employee will comment, or if you have a Pro License, you can open a support ticket.