I'm experiencing the same issue. Our SFDC campaign has about half the members of the Marketo program. However, the leads that were not added to the SFDC campaign do exist in Salesforce, so the sync issue is clearly not at the lead level but between program/campaign. The campaign was created in Salesforce, and the name was not changed.
Bumping this thread. This issue still persists and it seems like it's even worse right now. I'm having campaign members in Marketo not sync to SFDC campaigns until 8-10 days after their membership date. I've been going back and forth with Marketo and SFDC support. Nobody seems to know what's happening.