An SQL update query works fine unless it is invoked by an SSRS subscription. Why?

A stored procedure that updates data in production data tables was written by a very talented staff member (with developer-only rights) and works fine when executed in SSMS. However, the same SP does not run when it is invoked by an SSRS subscription. The subscription was created by and is owned by a manager with System Administrator rights. If this problem is caused by SSRS not recognizing the proper credentials (whatever they may be), how can we change the subscription so that it works? If it's not caused by incorrect authentication or authorization, what might the cause be? Thank you for your responses; they are very much appreciated. The "very talented" reference is a (not-so-obvious) joke and self-reference. SQL SMSS is version 2014, SQL DB Server is 2005, and the SSRS server is...2003. We use the subscription to perform a weekly update to customer records, changing/increasing their credit limit and terms if they are in good standing. The SP and report works fine when invoked directly by either the author or the manager (administrator). But the subscription, created by the manager, does not perform any update in the report's SP. So the question might be, "will a subscription created by the developer work?" Or, "does the authentication to the report's data-source need to be changed?" Or, "how can the SP be changed to permit its invocation from the SSRS subscription?" Or, "if the subscription is using the credentials of the SSRS service, how do we change it?"