Installed new flow master valve

I installed a new flow master valve in my toilet tank. It's the one that has the cup that rises with the water and then eventually shuts the water off to stop the tank from filling.
It works well, however, I have an extremely small leak and it appears that it's at the point where the valve stem beneath the tank meets the water supply coupler.
I can let it go all day and only have a couple of water drops on the floor, but it's driving me crazy. All my connections seem to be tight.
They did provide a cone washer that was supposed to go inside their water supply coupler, but I didn't use their coupler as I already have a flex pipe with a coupler on the end of it coming from the water supply pipe. So all I did, was drop their cone washer into my coupler.
I noticed that my coupler had the flat round washer in it, like the type you would find in a garden hose sprayer. I just realized that I didn't remove that washer, I just put their washer on over it. Could that be the reason for the small leak?
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