So whoever knows the right thing to do and fails to do it, for him it is sin.  James 4:17
And when the people complained, it displeased the Lord.  Numbers 11:1

All prayer which has ever brought down blessing has prevailed by the same law of success-the inward impulse of God’s Holy Spirit. If, therefore, the Spirit’s teachings be disregarded or disobeyed, or His inward moving be hindered, in just such measure will prayer become formal, or be altogether abandoned. 

Sin, consciously indulged, or duty, knowingly neglected, makes supplication an offence to God.

Written by A.T. Pierson in George Muller of Bristol

Disobedience in any form, a murmuring spirit, hesitancy to enter a pathway or duty divinely opened-all these grieve the Holy Spirit and effectually hinder our prayers.  Everything indeed that comes into our lives with which the Holy Spirit has no sympathy is a hindrance to our prayers, and we can have no real liberty until the evil is definitely renounced.
Taken from February 9 – Into His Presence – Daily Devotions for Prayer