Remember that sending International SMS requires TextMe credits.
Please check that you are typing : + (country code) (number) to text international numbers.
1. What is the model of smartphone you and your contact use?
2. What is the network operator (Verizon, AT&T, ...) that you use ? that your contact uses?
3. What kind of mobile network were you connected to (wifi, 3G, ...) ?
4. Are you sending messages to another user or to GSM? From which Country to which Country?
5. If you can send us the number or username you try to contact, it will make easier for us to investigate.