You must be accumulating a lot of data in your plan. Your data will roll-over, but any data that is not used for 3 months, will expire.
Here are the things to check in order to use Fizz mobile data:
1. Make sure your phone is compatible with Fizz network for data
Your device must have dual-frequency support for 1700/2100 MHz (also called AWS or band IV)
2. Make sure APN is setup properly. Restart phone.
3. Enable data on your phone settings.