Comment on page
Go to Settings > Object Manager, then choose your target object (e.g. Quote).
Go to Page Layout and enable Work with Salesforce Mobile Lightning Experience.
Once enabled, go to Buttons, Links and Actions to check for the button you added, and manually add it to the layout. You should only be required to do this once.
Adding buttons to a layout uses the Salesforce MetaDataAPI service. This service can take some time.
If this continues, you can try the solution above for adding a button manually.