Here is a list of some items that you should check in SFDC if you see a failed to sync error. Typically the root cause is linked to the permissions of the Marketo user that you have set up in SFDC but sometimes it may be a validation rule causing the issue. Usually the Marketo notification will show at least one of the leads affected which you can use then use to troubleshoot the issue.
- Has the SFDC Admin changed a user or field permission on the Marketo user profile?
- Has the SFDC Admin added any new field validation rules?
- Are there any look-up or master detail fields on the object in question? If these types of fields are being updated then make sure that the Marketo user has access to the object.
- Do you use record types? If so, make sure that the Marketo user has access to all required record types.
- Do you use Apex? If so you may have a trigger that fires on the update of a record, you will need to make sure that the Marketo user has profile access to the relevant Apex classes.
- Make sure "Convert Leads" is turned-on within SFDC. If it is not, you will get an error message when trying to merge leads that exist within Marketo and SFDC.
Here are some additional resources for learning about the Marketo integration with SFDC.