What lesson did Kisa Gotami learn the second time that she had failed to learn the first time?

Kisa Gotami understood that death is common to all and that she was being selfish in her grief. She understood this only the second time because it was then that she found that there was not a single house where somebody’s beloved had not died. 

At the first time, she was only thinking about her grief and was therefore, asking for a medicine that could bring back her son. At second time, when she met Buddha, he asked her to get a handful of mustard seeds from a house where no one had ever died. He did this purposely to make her realise that there was not a single house where no beloved had ever died, and that death is natural. When she went to all the houses for the second time, she felt dejected that she could not gather the mustard seeds. Then, when she sat and thought about it, she realised that the fate of men is such that they live and die. Death is common to all. This was what Buddha had intended her to understand.