Sorry to hear you have been experiencing issues with your flask leaking.
They are a bit quirky but can also be faulty. Before reaching out, please try out the below steps before getting in touch.
1) Turn it around very slowly
The flasks use a bit of science....
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.
Not usually an issue until you turn it upside down!
As these are designed to be turned around, 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.
The trick is to 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
I have had a few customers tell me that they find the filter part works best one particular way. If you look, you'll see the rubber is slightly thicker on one side. They say that the thicker rubber seal should face the water container, the thinner side should face the loose leaf tea.
If the seals and the filter are dry this may help "seal" the air in, too.
Also 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.
I recommend running the flask under warm water before filling it to help reduce the air expansion.