The cruelty of the heart prevents acceptance of Dua.

Meaning whatever a person asks God with the spirit which is hard and stony toward his brother in religion will not be accepted. It is narrated from Imam Sadeq (AS) who said: Allah (SWT) does not accept a request which there is a stony heart behind it. If your Dua is accompanied with a soft and kind heart, your request will be accepted, otherwise if you have a hard/rigid heart in regard to your brother in religion, then it will not [1].

[1] Akhlaq-e-Rabbani, Ayatollah Sheikh Mojtaba Tehrani, Vol. 8, Page 185.

Source: en.shafaqna.com