Christmas light stirng problems

I opened a new box of 200 Christmas mini lights (not LED) and 50 in the middle do not work. There are no missing or broken bulbs. I checked the 50 unlit bulbs to make sure they were seated tightly and they are. Do I need to check the ones that work? I know this is not a big problem in the scheme of things but I already put them up on a large outdoor tree. Would this indicate some sort of individual bulb problem or a fuse problem or a wire problem? I can reach the 50 unlit bulbs and the plug as fortunately they are on the lower part of the tree. Thanks if you have any suggestions.