The Recall Rate is low even when the FP==100

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I tested the Fast MTCNN on FDDB dataset, the minisize was set as 20, The max False Positive Value is 152. But most of the result on FDDB can reach 2000+ and racall rate is larger than 90% , what should I do to improve the Recall rate even if the FP arise ( means I need to detector more face and I could tolorate more error).
1. Improve/Decrease the threshold of NMS ?
2. Adjust other paras?
Looking forward to your Answer, thks