Agents take up to 24 hours to respond. Contact them here: https://giffgaff.com/support/ask
However, the community can help you as well if you explain your problem in more detail.
Other than the community, the agents are the only one's that can help. You can always ask an educator if your having trouble with agents, they might help, http://community.giffgaff.com/t5/Help-Support/Educ