A narrow opening can limit how wash water reaches internal surfaces, even when the container material is dishwasher suitable. The model's loading arrangement cannot remove every geometric restriction.
Understanding this situation
A clean exterior does not prove that the inside has been reached. Bottle makers may specify a separate method or a compatible appliance accessory for particular shapes.
A useful next step
Follow the container's own care instructions and inspect the result appropriately. Do not assume extra detergent will reach a sheltered area. If the item cannot be positioned with adequate access, use another suitable cleaning method rather than diagnosing a general machine failure.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.