Are you using the updated tutorial? This is the one to use: http://usegalaxy.org/galaxy101
When joining by columns, the content must be an exact match for a successful join. My guess is that there was a mixup during the execution of one of the prior steps in the tutorial you are following or incorrect columns were selected (this is easy to do when learning). As you go through the lesson, examine the dataset contents versus the tutorial graphics.
Once working on your own and using the Column Join function, verifying that the selected content is of the same type will ensure that if common keys are present in the selected fields, the data will be joined correctly.
Thanks, Jen, Galaxy team