If you think the lid seal is not tight, please check first of all whether the seal is twisted or distorted and whether it is in the right position.
You should also check that the rim of the mixing bowl is not distorted.
If the seal is cracked, you must replace it (click here)
Note: if the lock on the Thermomix ® TM5 is not working, it is important that you do not put a lot of pressure on the lid. This can damage the lid seal.