On being asked, "What is the reason that our prayers are not accepted?", Ibrahim bin Adham (a walyullah) answered because:
1 – You recognise the Lord, yet you do not follow and obey his commandments
2 – You study the scriptures yet you do not try to do what you are asked to do.
3 – You enjoy the gifts of your Lord, but you do not offer gratitude to him.
4 – You do not act so to avoid hell and enter heaven.
5 – You know the devil is your enemy, yet you do not deal with him as such.
6 – You know death must overtake you, yet you make no preperations for it.
7 – You consign your dead parents to the grave and yet you do not take lessons from the incident. (and feel as if you are immortal)
8 – You know you have many faults, yet you try to find faults with others
(Taken from Tadkharatul Awliyah (Memoirs of Saints)