iMessage issues

i have iphone 4. 2 days ago sent imessage 4s. sent text instead although have settings enabled not send text when imessage unavailable. yesterday imessage showing green in lieu of blue. @ home have excellent coverage, i'm not sure what's going on phone.

 

perhaps person you're sending didn't have coverage @ time?
in case believe iphone 'drop back' sms ..

 

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