Why are my sink shut-off valves leaking right at the wall connection?

Those are Accor FlowTite valves and they are absolute garbage. Most builders use them becuase they are cheap and fast to install, but they fail constantly. Pushing them further onto the pipe is not going to fix a leak if the internal O-ring or the plastic teeth have alredy failed. In fact, messing with them while they're under pressure might just make the situation worse. You definitly want to cut those plastic wall flanges out of the way. They’re just cosmetic and right now they’re preventing you from seeing if the leak is at the valve seat or if water is tracking back from inside the wall. Use a pair of side cutters or a utility knife to carefully snip them off so you can get a clear look at the actual pipe connection. Since you've got PEX piping, your best bet is to replace those plastic assemblies with a **chrome-plated quarter-turn ball valve**. If there is enought pipe sticking out of the wall, you can cut the old plastic valves off entirely. Don't try to pull or twist them off; just cut the PEX cleanly with a dedicated PEX cutter about an inch behind the valve body. You’ll need a 1/2-inch PEX crimp tool and some stainless steel cinch clamps or copper crimp rings. Slide the ring on, push the new valve into the PEX, and crimp it down. It’s a much more permanent fix than any push-fit or integrated plastic unit. If the pipe is too short, you might have to carefully remove some drywall to gain more room, but usally there's just enough slack to get a standard valve on there.

And don't bother with more push-fit stuff if you're already in there. A crimp connection is way more reliable for PEX. Just make sure the water is off at the main and you've drained the lines before you start cutting. Once the new valves are in, you won't have to worry about those plastic stems snapping or leaking again.

Ugh, those integrated supply lines are the absolute worst. Since the hose is built right into the valve, if that hose kinks or starts leaking, you're stuck replacing the whole unit anyway. If you don't wanna drop cash on a crimp tool, you coud just grab some SharkBite angle stops. They're a bit pricier but miles better than that plastic junk. Just make sure the PEX is cut straight and you've deburred the edge. Honestly, anything’s an upgrade from those FlowTites—I've seen 'em pop off and flood entire bathrooms in no time.

I miss the days when we’d sweat a solid brass stop onto copper and it’d still be bone-dry forty years later. This plastic 'push-to-click' stuff is exactly why I don't trust new construction.

When you go to replace them, check the pipe depth. I’ve seen builders leave so little PEX sticking out that you can barely get a tool on it. If it’s tight, I usally reach for a close-quarter crimper. It’ll save you from having to hack a giant hole in the back of your cabinet just to get some leverage on the crimp rings.