Hey, I was having the same problem and I thought it wasn't working either. I tried searching the same thing and found your issue as well. 2 days later, I realised that the problem was in my execution, specifically with the use of the 'detect' function.
I was using the ACFObject Detector before, and later tried using YOLO for my work. The 'detect' function in ACF works with a threshold range of [-1 1] and has influence on processing time and accuracy of results. You will get results with both -1 and 1 as values (however at -1 the results will be only slightly better). For me this value was set at 1.
Whereas the 'detect' function with YOLOv2 implies that an actual threshold in the range [0 1]. Any scores below this threshold wont be displayed at all. This has nothing to do with processing time unlike the ACF. Since I had the threshold set at 1, I ofcourse couldn't get anything to appear in my detection. As soon as I set the threshold to 0.5, my mean average precision was 0.86 because now it only required scores to be higher than 0.5.
I dont know if this is the case for you as well but, for me it was a simple mistake of not realising that the Threshold property in ACF and YOLOv2 works and implies in different ways.
Hope, it was helpful!