Why do I have a leaky tent?

When canvas tents are manufactured, the seams are sewn together using heavy-duty industrial sewing machines which use a lot of force to puncture a needle through the canvas and fill the hole with thread. The result is a lot of tiny holes which are larger than the thread and can let in water. By soaking the tent the thread expands and the canvas contracts to fill the holes and eventually prevent further leakage. 

This process is called 'seasoning' (or weathering) your tent. See the related article How do you season a bell tent?

Generally, a leaky tent issue will go away after seasoning a few times however if it persists then you can use a product such as Iosso Seam Sealer to speed up the process.