Your Marketo program has successfully synced with a Salesforce Campaign and you have been sending email, but have not been seeing any history sync to the Salesforce Campaign.
Only when there are Marketo program members will activity sync to SFDC, so your first step should be to check the Marketo program membership. If there are no members of the Marketo program, then that will signify the issue is just that.
Depending on the program and its setup, there may be lead activity that is not being associated with the Marketo program.
For example, imagine there's a default program synced with Salesforce. In the program, there's a child asset smart campaign sending emails, and you aren't seeing this email activity in the SFDC campaign. The child asset smart campaign needs to have a Flow Step: Change Program Status [for the specific program]. (It is recommended that this is flow step #1). Even though the smart campaign is a child asset of the program, these 2 assets are still independent of each other, so the flow step is needed to sync the activity.