Messages

If you are sending to an individual that has an iPhone and they do have iMessage turned on, it could still be sent as green if they do not have a data connection. This could be because their device is in Airplane Mode.

If it is happening to multiple people, then on your device, switch off/on the iMessage setting at Settings > Messages > iMessage. It would also happen if you do not have your Cellular Data switch turned on at Settings > Cellular > Cellular Data or your device is in Airplane Mode.

If you are using Group Messaging and a person in the group message is not using an iPhone or iMessage, then the messages will be green. If they are all using iPhone, then the messages will be blue.

In order for a message to be sent/received as an iMessage, both parties must be logged into iMessage and must have an active data connection. If everyone you message with is suddenly switching to SMS, that would indicate to me that you are losing your data connection for a brief time.