imessage not working from some wifi locations

I have an IPAD2 and my children have Itouches all setup for wifi (only). If in our home wifi network we can scew around and imessage and f2f etc, with no trouble. But now twice when I have gone to starbucks or to a hotel, I have not been able to imessage or f2f with them. I get cannot deliver message. But yet when they try to imessage me.. i can see the bubble indicating they are typing, but the message is never delivered. When I type they do not see anything nor get the message. We both have full access to the internet and email is working.. it is only these communication services (IOS5 on all devices) that are not working. Any clues as to what sort of settting I could be looking at.. I have changed DNS settings and NAT settings.. but nothing has helped thus far.

Thanx