Coffee cooling faster than expected does not by itself prove vacuum failure. The bottle may have started cold, been partially filled or opened repeatedly. The more useful clue is a clear change from the way the same container used to perform under similar conditions.
Compare it against its own history
Repeat a test with the same amount and starting temperature you used to use. A large, reproducible drop deserves investigation.
Touch the outside with care
In a working vacuum container, the outer body should not behave like a simple conducting wall. If filling it with hot liquid makes parts of the body unusually hot, contact the manufacturer; do not run tests that could burn you.
Separate a lid problem from an insulation problem
A poorly closed lid, a shifted gasket or a damaged part can let heat escape without the body’s vacuum chamber having failed.
Knocks and visible damage
A cosmetic dent does not automatically mean lost vacuum, but a major impact followed by a clear drop in performance is useful information for technical support.
Condensation where there was none before
New external behavior, together with worse performance, is a reason to review the product and the warranty instructions.
Do not try to repair the vacuum chamber at home
The double-wall body is not a household maintenance part. If you suspect a structural failure, contact the manufacturer.
Document the test
The model, approximate purchase date, photos and temperature measurements help a claim much more than “it does not stay hot like before.”
Practical takeaway
Change one variable at a time before replacing the bottle: cleaning, gasket position, fill amount, starting temperature or use pattern. Follow the exact model instructions. For perishable food and drinks, measured temperature and current food-safety guidance matter more than how warm or cold the container feels.
