When running a charge in Salesforce you want to pay close attention to the response message. Here are some situations that could often be the reason.
Gateway message was something different than declined. The error message is Salesforce saying they can't save the Transaction due to the Chargent Order being unable to be updated. There was an issue communicating with your Gateway
Chargent saves the Transaction Record on the Chargent Order. If another user is updating the same record at the same time or the fields contained a certain value then the record would not be able to be updated.