Why Didn't Narada Deal With Kamsa Properly

“Narada warned Kamsa to be careful of the friends and well-wishers and all the demigods taking birth in those families. Kamsa and his friends and advisors were all demons. Demons are always afraid of demigods. After being thus informed by Narada about the appearance of the demigods in different families, Kamsa at once became alert.” (Krishna, The Supreme Personality of Godhead, Vol 1, Ch 1)