Bathtub overflow gasket leaking

I've probably posted about this before but we've got a leaky overflow gaske t on our brand new tub in our brand new bathroom.
Our daughter just took her first bath. I pleaded with her not to fill it t o the overflow but she did it anyway. (She's 15 - need I say more?) Sure enough we got some water coming into the basement. Below the tub I now hav e a little 14 x 14 hatch that I can open up and inspect the tub drain and o verflow and do repairs if necessary although it is kind of tight getting up to the overflow.
I can see a nice brand new gasket up there. I'm kind of surprised it leake d.
I did have to snake out the drain once since the new tub was installed beca use it backed up due to unrelated issues. We have one section of old pipe that gets clogged every 6 months or so and the only place I can snake it ou t is through the overflow. The kitchen sink feeds into that same section b ut the tub is the low point so when it backs up it comes into the tub.
Is it possible I didn't tighten it well enough after I snaked it out? I ti ghtened it pretty good. I didn't want to bend the metal.
Could I have broken a seal when I snaked it out? It looks nice and tight w hen inspecting from below.
This is driving me insane. She loves baths and she likes the water level h igh.
Is it possible to get this thing working properly or are they just prone to leaking.
I still haven't made my last payment so I think I can get the plumber back here to look at it. I just want to be informed before I make that request.
Thanks in advance for any advice on this.
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