Currently Being ModeratedFeb 23, 2013 11:02 AM (in response to Vicki2006)
I presume you are using iMessage.
Messages can only go to iPhone users who have enabled iMessage themselves. Some iPhone users hate iMessage (don't know why, but my son hates it). That might be the reason.
Currently Being ModeratedFeb 23, 2013 11:04 AM (in response to Vicki2006)
And some people block messages from certain other people.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8