Google contacts not syncing on Android: fixes in order
Quick answer: Contacts that won't sync are usually saved to the phone instead of your Google account, or contact sync is off for that account. Check where the contact is saved, then turn sync off and on for the right account.
Video not loading? Open the video on YouTube. It may not match your exact model; check the written guidance and device scope before following it.
The fixes, in order
1Check where the contact is saved
Contacts saved to the phone itself never reach your Google account. Open the Contacts app, open a contact that isn't syncing and tap Edit. The account shown at the top tells you where it lives. If it shows the device rather than your Gmail address, that contact only exists on this phone. Save new contacts to your Google account from now on.
2Make sure you're looking at the right Google account
If you have more than one Google account on the phone, contacts may be syncing into a different one. In the Contacts app, tap your profile picture at the top right to see which account is selected, and switch to the one that holds your contacts.
3Turn contact sync off and on
Open Settings, then Passwords, passkeys & accounts (called Accounts on older Android), tap your Google account, then Account sync, and turn Contacts off and back on. This forces a fresh sync. While you're there, check the account isn't showing a sign-in error.
4Check your connection and battery saver
Sync needs a working internet connection. Battery Saver and Data Saver can delay background sync, so turn them off while you test.
5Clear the Contacts app's cache
Open Settings, then Apps, then See all apps, then Contacts, then Storage & cache, and tap Clear cache. This doesn't delete contacts saved to your Google account.
6Update the apps and restart
Update Contacts and Google Play services from the Play Store, then restart the phone.
7Check contacts.google.com
Open contacts.google.com in a browser and sign in to the same account. If a contact is there, it's safe and the phone just hasn't caught up. If it isn't there, it was never saved to Google, so go back to the first fix.
Important
Don't remove your Google account from the phone to fix sync unless you know your contacts are on contacts.google.com: contacts stored only on the phone can be lost.
Which phones this applies to
Menu names are from a Google Pixel with recent Android. Samsung phones also keep Samsung account contacts; check which account each contact uses in the same way.
What to tell support
Keep a short record of your exact device model, software or app version, the symptom or error text, and which steps you tried. Note what changed after each step. You can print this page for reference.
Do not include passwords, recovery codes or private account details in public comments.
Sources
Video by FixIt Mobile on YouTube. See the linked sources for current manufacturer guidance. Menu names and available features can vary by model and software version. How we check fixes.