There are many components that can cause bitter vegetable broth. In fact, celery, one of the most popular additions to vegetable broth, can often cause bitterness if you are using leftover scraps. I would recommend sticking to fresh celery when making broth, but if you have scraps you'd like to use up then stick to only the white portion. The green portion of the stalk becomes bitter as it ages, and be sure to not include the leaves before adding it to the pot. The onion and carrot can be another culprit if you've included the skins. Be sure to peel the skin on both and discard the root of the onion.