iphone not syncing all media from computer

How can I get all the songs from my Mac to sync on my iphone? Currently, the music app on my iphone contains 29 GB of songs, whereas my Mac has 118 GB. (The songs I want to sync are all "ticked selections" in the Music app.)

On my iphone storage screen it shows "81 GB of 256 GB used" – 175 GB of space – meaning there is plenty of room for whole the 118 GB of music to be synced to my phone.

(Up until recently the sync was working fine – all 118 GB was syncing from Mac to phone)

Thank you