I bought extra iCloud space that’s not full but my phone is always full

My iPhone says my storage is full, but I pay the $11 extra a year for 20 GB of iCloud storage. I’m only using 6 GB of my 20 GB in the iCloud, but I’m still unable to back up my phone and am still getting message my phone be is full. What’s the point of the extra iCloud if my phone is always full?