The reason is that we don’t do sincere remembrance of our worshipped form of God; we don’t love Shree Krishna. Since we don’t love Him, inevitably, our heart doesn’t melt and as a result, we are overcome by slackness and sluggishness.
Sit up straight and be fully alert while doing devotion. Stand up if laziness or drowsiness starts to become an obstacle or causes a hindrance. The condition however, is that one should maintain loving remembrance of Shree Krishna.
Sing and chant the glories of your worshipped form of God. Always remember that one may be attracted to a particular virtue or attribute of another person. Just as in the world, an individual may be intrigued by a particular virtue of another individual. Beauty is also a quality; likewise, adham-udharanhar (uplifter of fallen souls), patit-pavan (purifier of sinful souls), bhakt-vatsal (One Who has deep affection for His devotees), etc, are the uncountable virtues of Shree Krishna. Constantly remember and meditate upon them.
Just the chanting alone of the virtues of Shree Krishna isn’t enough. Even singers and musicians sing the virtues of God, but they don’t receive God realization. Thus, while chanting the glories and virtues, it is important to evolve feelings of love and affinity accordingly. For example, (while practicing devotion we should develop such feelings that) as individuals, we are fallen, sinful, and carrying a huge load of uncountable sins on our head, but Shree Krishna is causelessly merciful, has deep affection for His devotees, and is the purifier of sinful souls, the uplifter of fallen souls and so on.