If you have unconverted Leads that aren't flagged for match review and have no Campaign associated with them, there are two potential explanations:
1) The Soapbox Engage form does not have a Campaign associated with it
This is the simplest explanation and the easiest to check. To do so, open the form in your Soapbox Engage administrator and confirm whether or not it has a Campaign selected.
If a Campaign is listed as being associated with the form, you can double-check that this Campaign is still a valid Campaign in Salesforce by clicking the Select box and confirming that the Campaign is displayed and selected in the popup. If it is not, it may have been deleted from Salesforce. If so, you will want to select the Campaign created to replace it or create a new one to associate with the form in Soapbox Engage.
2) A Salesforce error occurred upon attempted Lead conversion which caused the Lead conversion to fail and prevented the Lead being added as a Campaign Member
If a Salesforce error is triggered by the Lead conversion process, Salesforce will revert back to the original Lead record and remove the association with the Campaign. Generally, this sort of error is caused by something unique in your Salesforce setup. Common scenarios are validation rules on the Contact or Account (Organization) object which would prevent Lead records from being converted if they don't not meet the criteria.
To troubleshoot this, review the validation rules you have in place for Contacts and Accounts to see if the Lead record in question does not pass those rules. If practical, you may wish to disable these and attempt to add the Lead record as a Campaign Member of the desired Campaign. This will trigger the conversion process and allow you to confirm if disabling the rule resolves the issue. If the issue remains, you will be able to see the text of the error to inform further investigation.