In addition to the usual reasons, message delivery failures may also be due to the new restrictions on sending SMS to US numbers. Please ensure that you're complying with the latest requirements, which include having a US Skype Number set up as your caller ID if you're attempting to text US numbers. Refunds: Users who were charged for SMS that were not delivered due to this block will receive refunds.
Also, verify the recipient's phone number and that you have enough Skype credit, then try sending the message again.
If you’re sending a text to someone located in the USA, only the following carriers support receiving Skype SMS text messages: Alltel, AT&T, Cellular One Dobson, Cellular South, Centennial, Cincinnati Bell, Nextel, Ntelos, Sprint, T-Mobile, Unicel, U.S. Cellular®, Verizon Wireless, Virgin Mobile USA.