How do I force text messages to send and receive via iMessage?

You can delete the SMS messages from the conversation and just leave the iMessages. Then reset your phone using the sleep/wake + home buttons. This has restored the default to Send via imessage again for the conversation.

Try disabling iMessage in Settings → Messages → iMessage. Turn it off and then turn it back on. After doing this, close all apps and restart your phone by holding down the Lock button and the Home button for ten seconds. If this does not resolve the issue, it may related to the other users' devices.

You can also try restoring your iPhone though iTunes and setting up as a new device if you don't believe it's related to the other users.

If you're using Sprint and Google Voice, see this support article from Apple: http://support.apple.com/kb/TS4091

Summary:

Integrating your Sprint account with Google Voice may prevent your mobile phone number from being registered for use with FaceTime video calls and iMessage text messages.

This occurs only when your Sprint phone number and Google Voice number are not the same. It should not affect your ability to send or receive SMS or MMS messages, or to receive or place FaceTime calls or iMessages using an email address instead of your phone number.

The support article doesn't give a solution, but I infer that you would need to change something with respect to Google Voice. I don't have a Sprint account and I don't know what it means to integrate a Sprint account with Google Voice.

I just had the same issue (also with a person named "A") and was able to resolve it by sending an iMessage to A.'s email address, which she had registered to receive iMessages. She received it fine. That seemed to wake her iPhone up to the fact that I was sending from an iMessage-capable number, as she was then able to send me an iMessage to my mobile number, where just minutes before it didn't work.

We then repeated the procedure in reverse (she iMessage'd my email address). Now we are both able to send each other iMessages to each other's mobile number.