My imessage is not showing me a text box to respond in.

I have a group of two people that I text with every day.

The three of us send text and pictures daily.

About a week ago, I could not respond to that text group because the text box does not appear. Therefore no keyboard is prompted.

All of my other text groups work fine.

I can force a response to that group by going to a photo, then sending it to that group.

That action initiates a new conversation to the group. The other group members see the response as a normal response in the conversation thread.

After a few minutes, (I suppose when iMessage performa the synch across my Mac, iPad, and phone) the same problem returns and I am no longer able to reply to that text group.

Any ideas would be greatly appreciated.

I went to the Genius Bar and they wiped and reloaded my phone. It seems to have worked, but after I left the store, the problem returned.