This is seeming to be a problem for a lot of users. I figured it out by going to my iPhone Settings, signing out of iMessage, then sign back in. Make sure when you sign in both your phone number and e-mail are checked for iMessage then restart your iPhone and Apple Watch and all should be good. This is just what I did and now everything is working perfectly. I was getting messages but all of my sendings ones had failed until I found my own solution. Let me know if this works for you.