You can check the following common reasons:

1. the Verify VAT Numbers checkbox in Accounting --> Configuration--> Settings is not enabled:

2. the country of the Contact in Odoo is not listed in the European Union Country Group in Contacts --> Configuration --> Localization --> Country Group:

3. A single character has been entered in the Tax ID field on the Customer:

4. The Country of the Customer is the same as the  Fiscal Country of your Odoo database, defined in Accounting --> Configuration --> Settings:

VIES is intended for verifying VAT numbers in cross-border transactions between EU member states. For transactions within the same country, the local tax authorities are responsible for validating VAT numbers. 

Example: a company in Germany wouldn't need to verify a German VAT number via VIES, as this is a domestic matter handled directly by the German tax administration.  When two companies within Germany trade, they are subject to regular domestic VAT rules rather than cross-border VAT treatment, so using VIES for these validations isn’t necessary.  Running VIES validation for local VAT numbers would be redundant since the fiscal authorities in the country already maintain this data and manage local compliance. This is why Odoo skips the VIES validation for partners with the same country VAT as the company's fiscal country.