Maybe it is because it does not have the right amount of rosin on the hair, or the fingers might accidentally hit other strings. It can also be because the hair is too tight or too loose. Also, if its location is too close to the bridge, it will get scratchy or be too far up the fingerboard; you will hear squeaking and lose the clarity of tone. Moreover, at times, it can be because it is too old; at such times, you will need to buy a new item to avoid such issues.