opencv_traincascade insufficient count of sample in vec file

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I have 20 positive images and 750 negative images. I generate the positive sample, it gave me a info.lst(37800 line records) and newly generate image in the info folder. It has around 37800 images. After that I start generate the vector file. With this required info ready, I start train the classifier using script below

opencv_traincascade -data data -vec positives.vec -bg bg.txt -numPos 37000 -numNeg 700 -numStages 40 -w 20 -h 20.

It gave me an error when in the stage 5. It complaint insufficient count of sample? Why it complaint insufficient sample since I have 37000 newly generate positive sample. Please help