This actually happens frequently. In many cases it happens even though a 911 call brought LE to the house. 911 hang up call, LE goes to the house and there is no answer. They figure it's a prank call and leave. There was a case here in Utah several years back where a baby died of starvation in it's crib after mom and dad died in a murder suicide. LE had been at the house knocking but never went in. Two more cases I can think of where it was murder suicide discovered by family breaking in when LE wouldn't enter. One of those was a 911 hang up call.
In the Entwhistle case, the mother had to insist that LE enter the home to check on her daughter. They did enter, looked around in all the rooms and reported that nothing was amiss, even though there were two dead bodies on the bed in the bedroom!
I bet LE gets tons of welfare check calls, and over time they just get jaded. I guess if you'd done, say, 1000 welfare checks over the years and no one ever turned out to be in trouble, you just wouldn't put much effort into it.
ETA By "you," I don't mean you, personally, SuziQ!