I'm familiar with the Cvent integration, as I work with some clients like yourself. There are a few things that could be going on here, but I would need some additional information to help troubleshoot. Can you let me know the process you're using to create these new custom fields? Are you getting an error(s)? I'd love to help, just need a bit more information to properly do so.
Yeah, can you explain more about what you want to do, with details on fields, and screenshots?
I believe cvent has alot of fields you can sync and it is important to allow those fields to be synced and to map them to fields in Marketo. You may also need to define how and when those fields are updated.