Too dense sourdough bread occurs due to insufficient yeast activity in the dough.
The initial cause could be under-proofing.
If you don't give enough time for the wild yeast to multiply and reach its appropriate population where it can cause enough leavening, your bread will become dense.
Another possible reason could be because the yeast has died for reasons such as, chlorinated water or bleached flour that still has a high percentage of bleach residues.