That must be a heavy bass earphones but it is not enough, why?

Betron B750 is designed for low frequency performance and it produces heavy bass. However, noise isolating earphones can only perform when they fit your ear perfectly and seals your ear canal. Otherwise the speaker pressure will leak and you will not be able to hear the bass frequencies. Please try all earbuds provided with your earphone to make sure you find the best fir for your ears.