I recently put in a full bath in my basement (shower, toilet, and sink). The plumbing was redone and venting was connected to existing venting. The drains were all tied to existing waste pipe.

After using the shower once I realized that I had a vent issue as the toilet gurgled. I disconnected the vent where it was connected to the existing vent and the problem went away. I realized that my old vent must be clogged so I installed a studer(?) vent. This seemed to fix the problem.

Just the other day I ran into the same issue when someone was taking a shower on the second floor and the basement at the same time. The water in the basement shower would not drain and the toilet gurgled. They do share the same waste pipe. This is tied in below the basement floor. The basement floor is roughly 7 ft below grade.

What do you think may be the issue? How would you recommend fixing?