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Weird toilet smell

We have a strange situation where we’re getting a bad smell in our bathrooms (started with one, now two toilets). The smell comes and goes, and can be stronger on some days. We’ve previously had a plumber coming to check, toilet has been taken off the wall and examined behind, also checked and did some work on the roof with the vents (extended to avoid sucking the bad smell in). But the smell is still there! If anyone who’s experienced, could you let me know how you’d approach this situation? What could be the cause(s) and how you’d exam and fix it? Thanks!