Drive tension in totally dependent on the spring tension and making sure the belt is the correct length.
Sometimes the belt gets worn (people say it stretches, but it does not).
But since the belt gets narrower, if fits further down into the V pulley.
By sitting further in the pulley, it does not have as far to travel. Because it is the same length, then it is "too long" for the distance it travels.
Also there will be wear on the back of the belt, so the iddler pulley has to move further to make contact.
Check the belt and make sure it is not on the worn out side..
Also check the condition of the pulleys and the spring that applies tension to the idler pulley.
See if there is ANY way to make the spring to have to stretch further (by adjustment).
If there is no way to stretch the spring further, then you will probably either have to replace the belt or replace the spring or both.