Just because they have iPhones, it doesn't mean they have iMessage enabled. Also, if the recipients do not have their phone number selected to receive iMessages, you need to find out which Apple ID they are using for it. Having said that, if you've made sure that the above are fine, go to Settings->General->Reset and then select "Reset Network Settings and then try to send an iMessage to one of the recipients who were unable to previously receive messages from you.