I have found that the battery holder is at fault sometimes. I have had to add a dab of solder on the + end of the battery to make contact with the terminal. Once that was done it worked like a charm. I use EBL protected 18650 3000 mAh Li-ion batteries. I think they may be slightly shorter that some of the others. Anyway...... that was a long winded way to say check your voltage at the terminal on each end of the battery holder with the battery in place....