iMessage operation changed on my MacBook

6 months ago when I got my new MacBook Pro I was surprised/glad to find the iMessage program, wherein it automatically recognized my iPhone and I started getting msg's on phone and MacBook (it was cool to not have to use phone keyboard to send longer msg's). Then, I went to europe recently and something has changed on MacBook...my phone and computer don't seem to sync WRT msg's, e.g., msg content that's on iPhone isn't showing on MacBook, also some people to whom I could msg from my MacBook before are now coming up with RedHighlight in "To:" bar and an error indicates , "this person not registered with iMessage" (even though they're on Apple devices).

Any idea how I can get back to the Good Ol' Days of having the ability to msg anyone I want from my MacBook? Thx