I doubt it’s really a design issue so much a defect in one or more manufacturing batches.  That would explain why the vast majority of Arc owners are not seeing the issue you are, but it’s more than just a one off case.

I waited 10 days before I called them back. Another apology, another promise to increase priority of my issue and another promise of an email in 24-48 hours. 

I had this issue at first but after reading the comments, I pushed a little harder and gave it a little wiggle and the adapter seated flush with with the housing. It's very secure once you get it snug! Attached a pic showing how it should look when snug and flush.  

I recently bought the Arc and was about to return it for this issue. After reading the comments I turned it over and used pressure to press the cord in and now it’s snug. 

I have the same issue and the Sonos tech support said it’s a “standard” cable,

so… I will have to suffer with it. The problem is that the power end of the cable is

made of hard, slick plastic, not softer rubber, so if you breathe on it, it falls out of

the socket. The Arc is a nice unit, but, give me a break and offer better power cords

that stay put. It’s made even worse, because the power socket is way up inside the unit

where it can’t be seen while mounted on the wall, so it’s nearly impossible to plug back

in without unmounting the unit. Unacceptable!

Is yours flush, like the picture above? 

If it isn’t flush try my suggestion (on page 1 here) to use a DRY silicon lube on the cord.

Just a light touch on the outside of the connector.

Update: 

I took the Arc off its wall mount and got the power cord end all the way in,

so it now looks like the photo above and it is now snug.

Note to all: It must be perfectly flush. It’s all or nothing!

Thanks for the photo!

The picture above (Nhopper) is what finally helped me resolve the loose cord issue. After a year of dealing with it popping out, I finally just got a pair of needle nose pliers and pushed the cord in...it went in nice and snug! I was pleasantly surprised that after all this time it was supposed to go in that far! Thanks for the photo! 

I had the same issue, so I took a torch to see what was going on. I noticed that there was a hole about the size of the power plug disk and the bottom of my disk was flush with the surface of this hole, but not actually fully plugged into the socket underneath. so I manoeuvred the disk around and it just fit into the hole with little or no tolerance around the edge. I was then able to push it in further and the disk top was then fully flush with the hole. It is now rock solid. Silly mistake I know, but easy to make if you are installing in low light. Hope this helps someone.

I had the same problem with my Arc, push hard the power cable and it will click.  You wont need a new power cable

I had the exact same issues and it was driving me crazy!  What I found was I thought the flange on the cord fits flush to the opening, however I did indeed need to push the cord in further so the flange actually recessed into the opening.  Once the cord flange was fully pushed into the opening, the connection was solid.  Now it is secure!

I was having kittens after spending so much money to find the cable kept disconnecting from my ARC. But thanks to this thread learnt that all I had to do was push really

hard on the plug to get it seated correctly. It is now rock solid.

The wide outer ring of the plug actually goes inside the plug hole on the ARC. It takes some real force to get it to click in, but once in, it then looks completely flush with the outside of the ARC body. 

I was having kittens after spending so much money to find the cable kept disconnecting from my ARC. But thanks to this thread learnt that all I had to do was push really

hard on the plug to get it seated correctly. It is now rock solid.

The wide outer ring of the plug actually goes inside the plug hole on the ARC. It takes some real force to get it to click in, but once in, it then looks completely flush with the outside of the ARC body. 

 

This is a terrible design flaw that Sonos is reticent to acknowledge,  I have had nothing but frustration trying to convey the issue to customer service,  they are in utter denial about the issue

What issue? The flange is what’s holding the connector connected. It can only do that when it is pushed in so the outside of the connector sits flush with the speaker.

Maybe (I didn’t go look) Sonos needs a better picture and instructions in the install manual?

If it solves nothing else it will at least let tech support say “It is in the manual.”

my plug is inside the hole as in the picture above but still nothing.  The center “hey sonos” light comes on orange or white for a second, then goes off.  The power light never comes on. Any other ideas?

“Center Hey Sonos” light? Isn’t the center light the power light?

If it comes on when booting and then goes off is there any possibility you set your Sonos to turn the LED off after booting?

Stanley_4 No it was the power light which is on the bottom left side. I checked to be sure I hadn’t set it to be off (unlit) but it wasn’t that.  It ended up I had to reboot the system after getting the plug in correctly, because I’d been messing with the Arc power.  Thanks.

Now I’m really confused, looked at my ARC again as well as the Product Guide and I only see two LEDs, Status and Mike.