A lot of waterproof fabric nylon items- tents, flys, tarps, etc, will leak at the seams unless they are sealed.
The reason they leak at the seams is because the nylon fabric doesn't swell when it gets wet, closing up the needle hole where it was sewn, like the older canvas did.
If you are getting leaks at the seams you need a seam sealer made for NYLON tents, not canvas tents.
A good camping supply/backpacking type store will have what you need. If there's none in your area, try
www.campmor.com
BTW, if the tent in question has a waterproof floor, check that floor for seams, too, and seal them.