The pedal assist on the Beryl e-Bike is capped at 15km/h. This means that once you start pedalling, the motor will assist you and help you to speed up until your reach 15km/h - the assist will then cut out. If your speed falls below 15km/h, the assist will kick in again. This may feel like the assist is not working, but the bike is designed this way to comply with regulations and keep you safe.
If you are pedalling very hard, you may not feel the benefit of the assist. Try pedalling more smoothly and let the motor carry you, rather than the other way around.