SMS messages not sending

Since a while a lot of the SMS messages won't get sent.
It just says that it could not deliver the message.
What I do then is copy the message to the clipboard with airdroid and paste it on my phone, but of course that's not ideal.Does anyone have a fix for this? Is it my phone, my network or Airdroid causing this problem?

Hi Prestodus,
Thanks for your feedback.

Would you please tell us more details about your issue? Such as the operations, the error message?