Uh, fire the janitors if they aren't doing their jobs. Plenty of others are looking for jobs and I'm sure some would even do that.
It's up to the individual who clogs the toilet to unclog it.
It's NOT the janitor's job, it's not the job of the young people in the office, nor the new guy...and while it may indeed be caused by an issue that requires a plumber, it still comes down to whoever clogs the fucking toilet needs to grab the plunger and un-clog the dammed thing.
It's the janitor's job to clean the bathroom, stock the tp, soap, ect, but not to unclog the toilet. That would be the responsibility of the landlord or property manager to either do it or delegate it to someone else.
Then fire the janitors and include this responsibility in contract of the new hires. You don't even have to fire them, just inform them one more duty is on their list and can quit if they want.