Connected strands not working

I just bought brand new philips 100 mini lights. I bought 8 boxes and tested each box before putting them on the tree. The box states I can connect up to 11 sets. However, when I plugged in the bottom strand only half the tree lit up. Every strand worked before I put them up so why is it now, 20 minutes later, not working? I changed the fuses in the plug (though it worked before but tried) and it did nothing. Any ideas?