MacBook air not in sync with iCloud photos

See the article Set up and use iCloud Photos - Apple Support 

On your iPhone, iPad or iPod touch

1. Go to Settings > [your name].
2. Tap iCloud.
3. Tap Photos, then tap the switch next to Sync this [device].

Download your photos and videos from iCloud.com

1. On iCloud.com, click Photos.
2. Select the photos and videos you want to download. 
3. Click and hold the download button https://support.apple.com/library/content/dam/edam/applecare/images/en_US/il/ios9-icloud-download-icon.png in the top right-hand corner of the window. If you want to download your content as it was originally captured or imported, choose Unmodified Original. For JPEG or H.264 format – including edits and even if it was originally in HEIF or HEVC format – choose Most Compatible.
4. Click download.

If you exceed your iCloud storage limit, photos and videos won't sync to iCloud and your library won’t stay up to date across your devices. To continue using iCloud Photos, you can upgrade your storage plan or reduce the amount of storage that you're using.

Note : it's not necessary the photos are still on iCloud server , they were not saved as iCloud storage was short , if you decided to upgrade the iCloud storage a sufficient space must be there , you need to check the status of iCloud storage Check your iCloud storage on any device - Apple Support

once iCloud storage is sufficient , if you upload new photos / videos they will sync with other devices ( iPhone / iPad ) Access and view iCloud Photos on your iPhone, iPad, or Mac - Apple Support in which photo library is set ( turned as on ) Set up and use iCloud Photos - Apple Support

Make sure photos are not hidden ,Hide photos on your iPhone, iPad or Mac - Apple Support 

Hide photos from view in Photos on Mac - Apple Support