There are a few reasons that this may be occurring:
If you are using Metabase 0.33 or above, you can perform joins using the Notebook editor. Note that you will need to choose a field to join on, or link, the two tables together. For instance, if you want to combine a Customer table with an Order table, you might select the ID field in the Customer table and link it to the customer_id field in the order table.