1) Turn it around very slowly

When you add boiling hot water to a sealed bottle, the air inside expands and will try to escape to release that pressure anyway it can.

It is important to let the hot water and air "swap places".

If you turn the bottle around too quickly, the air will be compressed further and will try to escape out the seals, causing a leak.

Ensure both sides are screwed tightly, and then to turn the bottle around very, very slowly - holding it horizontally until the water and air have had a chance to move into the opposite chamber - then finally all the way upside down. 

2) Turn the filter around and keep it dry

You'll see the rubber seal is slightly thicker on one side of the filter. The thicker rubber seal should face the water container, the thinner side should face the loose leaf tea. 

Ensure the seals are dry and check that no loose leaf tea is in the seal.

3) Use hot, not boiling, water

Tea is not supposed to have boiling hot water poured on it. It kills all the desirable nuances of tea and all you are left with is a strong flavour of dry, astringent tannins.

Stop the kettle before it boils or let it sit after to cool slightly. This will mean the expansion of air in the bottle will be less and won't try to "escape" out the sides. 

Run the flask under warm water before filling it to help reduce the air expansion. 

If you continue having issues please get in touch.