Hey there Zoe19!
I have a couple of articles that can help you troubleshoot this issue. First, if you are experiencing the issue on your Apple computer running OS X, you will want to see this article:
If you are experiencing the issue on your iOS device, you will want to see this article:
If the issue is consistent across multiple devices, then you will want to check on the settings of your router and match them to these recommended settings:
Thanks for using the Apple Support Communities. Have a good one!