On the iPad, are you connecting via wifi or cellular?
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
Thank you for replying and for support web links, in answer to your question, the iPad is connecting via wifi at my home address and my daughters mac is connected through a cable to the student accommodation broadband. The only thing that springs too mind is that she came home for the weekend and connected to the internet via wifi. The FaceTime hasn't worked using the ipad to mac since returning to uni.