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There could be several reasons why no error message is displayed when OpenCV VideoCapture fails to function:

1. Incorrect input parameters: Sometimes, when incorrect input parameters are provided, VideoCapture may not be able to open the file or camera. However, if the input parameters are not explicitly checked by the user, the error message is not displayed.

2. Silent failure mode: The OpenCV library has a silent failure mode, which means that some functions silently fail instead of displaying an error message. This mode can be enabled or disabled, but in most cases, it is enabled by default, which means that VideoCapture fails silently.

3. Exceptions are disabled: OpenCV does not throw exceptions by default, which means that errors are not reported. If the user has not enabled exception handling, VideoCapture will fail silently.

4. Improper error handling: Sometimes, application developers do not handle exceptions correctly, which means that the error message is not displayed even if it occurs.

To overcome this issue, developers should always explicitly check the input parameters and enable exception handling if necessary. Moreover, proper error handling should be implemented, so that if an error occurs, the user is notified.