Backwater Valve Stuck - Toilet Backed Up

Had a problem with all downstairs sinks draining to a toilet on the first floor. Toilet water level was increasing even when the water valve to toilet was shutoff and water level in the tank was not decreasing. Plumber came and worked on some things outside and said the backwater valve had sealed itself shut thus meaning there was no where to drain the downstairs water.

We just moved into the home Saturday, new construction, sat for about 5-6 months before we moved in. We have ran dishwasher a couple times, done dishes twice and flushed that particular toilet numerous times. Does this sound like the backwater valve was stuck from the beginning? Not sure how long it generally takes for things to start backing up if it was stuck. Just seems like we had a lot of water go through the pipes before it did start to back up.

Usually they do not get "stuck" closed. More often the "flap" does not open fully, which restricts the movement of paper, etc., through it and it starts to build up in the valve. Eventually, the mass will become large enough to seal the pipe and then you have the backup, but it can take days, weeks, or even months to happen. Since my opinion is that "backwater valves" cause more problems than they cure, I usually remove the mechanism from them so they become just another section of straight pipe.

It seemed like it was stuck because he even started to fall backwards as a result of his momentum carrying him once it became loose as he was pulling on it. It's weird!