Over time, and due to ongoing usage, the waterproof coating on your tent fabric may begin to wear off. A common sign of this is water being retained or transferred through the inside of the tent, rather than collecting or running off as usual. If this is the case, you will need to re-proof your tent.
There are many products available to help you to do this, such as tent and equipment proofer.
You will simply need to pitch your tent on a dry day, apply re-proofing spray and allow to dry.