We're sorry to hear you're having issues with your digital piano, however, this is usually not a fault.
Digital pianos are designed to emulate the sound of a real acoustic piano, which uses lengths of string to produce the sound.
As the higher-pitched notes use shorter strings, they resonate much less than the longer strings, resulting in shorter, less sustained notes.
You can confirm whether this is intentional on most digital pianos by changing to a string sound, which should sustain much longer as you hold the key down.
If you are having an issue with specific keys, please raise a support ticket and provide a video demonstrating the issue, so that our team can review the issue and advise further.