Q: I have problem with iMessage.

Your friend must be logged on to a wifi network in order to use iMessage.  It sounds like the first time that was true but the second time your friend was not on a wifi.

Yes, that's exactly right.  At the moment, iMessage only works over WiFi.  If you're in the U.S., there's some talk that AT&T will start supporting it in the fall.

Thanks and good luck.

By the way, when your friend is not on wifi, you can still send him/her text messages (at least in the U.S.), either by e-mail or by using the carrier's website.

For example, if your friend is on AT&T wireless with phone number (987) 654-3210, you can send an email to 9876543210@txt.att.net which will result in a text message.

WWhen I get a txt message it's not coming up on my phone to let me kno I'm getting a txt!

You might want to consider starting a new discussion. Since this one is a couple of years old, less people are likely to look at it. A new post would be much more visible. You can link to this one.

Handoff Continuity System Requirements

Handoff Continuity Troubleshooting

Handoff Continuity Troubleshooting (2)

iPhone, iPad, iPod touch, and Mac using Continuity - Connect

SMS Relay - Set up and use –Yosemite/ iOS 8

SMS Relay - Set up and use –Yosemite/ iOS 8  (2)

Text messages forwarding 

Messages – Fix Syncing Between Mac and IOS (2)