WCTP - Not sending to Sprint users


My company uses Enterprise Messaging to text message. Currently using WCTP and text messages send to Verizon, AT&T, T-Mobile, every carrier is fine except Sprint. If we send a text out to a Sprint user - they do not receive the text. According to Verizon, this is supposed to work for all carriers. Anyone have any knowledge of this? Is there something different that must be done for Sprint? Any help would be greatly appreciated.