Commercial lease obligations under NNN

[Andrew Shrigley]

I have recently leased out my commercial property. The lease states: 

1. Landlord agrees to make all necessary repairs to structural portions of the Premises, including the roof, exterior walls, foundation, and exterior sewer line.

When preforming agreed upon repairs all the toilets were flushing and operating, not clogging.
The tenant has been occupying the building for two months and emailed me yesterday saying that all the toilets were clogging. I recommend they call a rooter service to have lines snaked. They argue that it is my responsibility for bill because of the scale build up and sludge. It's under $500 so not an extreme expense but I argue that it falls under the NNN maintenance purview, responsibility of the tenant. There are no breaks in line just build up.
it’s two months into a 39month term. I don’t want to create riffs in our landlord/tenant relationship but I also don’t want to pay for it. 

In this case, who should be responsible? 

All comments and opinions on this are welcomed and appreciated. Thanks!