Do you have a texting plan? Do you know if the numbers that are working or not working are iPhone users as well. If you have iMessage active, texts will be sent via the Internet using iMessage vs texting (cellular) to other Apple iPhone users. Apple keep a database of these and routes them accordingly, but sometimes I find with poor Internet connections on my end or receiving end, message will not send. You can select to send these as a text message by holding down on message and a pop up selection will give you the option to send as a text message.