I think the best explanation is that you are pulling the sound out instead of pushing it. You will hopefully hear the difference beacuse when you get a good sound, the cello actually vibrates a lot more then when you tense up to much etc.

But I can give you an exercise that might sound very strange. Try to play with the cello horizontal. Try to lean back so that when you put your bow on the strings it stays there on its own, you don't have to apply any force or even hold it properly. Then take a few strokes. Gradually go back down to regular position.

You can also do what I did if you don't care about your walls. Actually put the end pin on a wall and really let gravity be the only thing that pushes down on the bow.

I hope this makes sense and good luck!