I wonder how many of you have thought you have had tension issues when actually, it was simply a case of your thread sitting outside of your tension discs rather than going between them.
Imagine two cymbals together, these are your tension discs. When your presser foot is down, the cymbals are tight together and you are not going to get anything between them. Raise your presser foot and they come apart. So, if you thread your machine with the presser foot down, your thread will not go between the tension discs.
If you thread your machine one handed and therefore have no tension on the thread as it is put into the tension discs, again, the thread may end up just sitting outside of them. The tip is to ensure your presser foot is up and to hold the thread under some tension when you put it into the slot where the tension next to the dial. This enables the thread to go between the discs and all will be well in the land of tension.