No suction. Air comes out of small vent.

To add to mayer's response. The air redirection is an indication of a clogged dirt intake (this is done to prevent burning out the motor) The re-direct commonly when the dustbin is too full.

In theory you don't want the Dyson's dustbin to get completely full. When this occurs the vac becomes slowly less effective as it packs more and more dirt into the bin, and more likely that debris may not be handled properly and resulting in the dirt intake becoming blocked.

More likely and in my case the dirt intake blocks when an item which is slightly longer or wider than the dirt intake channel airway can accommodate at one point or another. Problem areas are the U-bend, the bend just before the dustbin, and the accordion hose just after the brush bar.

Problem is that an item is able to be picked up by the vac and it doesn't immediately clog, and the user is unaware that the dirt intake is partially obstructed. Over time items catch on the same piece of debris obstructing the dirt intake and eventually cut the suction to the point where the suction redirect kicks in.

Checking these areas (and any other areas where a narrowing of the dirt intake occurs) you should locate a blockage and clear it. (I use a inexpensive piece of semi-flexible rubber hose to snake through mine to clear it when this occurs. (avoid metal or harder plastics as they may puncture or damage the intake channel)

Note filters when properly cleaned will allow you to see light (commonly a bright light will allow you to see a little light through each individual piece of the filter sponge or filter) through the filter media if it is properly clean and dry. (if you can't see any light at all clean and dry the filter again until you do.

Also ensure the Cyclone assembly is not clogged with any debris. Tapping on the side of the inside of a solid garbage bag is normally sufficient to dislodge accumulated dust, but because there are many small holes within this part it may have a lot of accumulated dust. Wiping the inside of the cyclone helps with the exterior dust but not the dust that has made it's way up into the chamber. So the tapping from different angles will dislodge this dust and it will "rain" out of the unit. Doing this also improves the performance of the filter between cleanings.

I'm having this problem on a 2015 model Dyson Animal upright vac which does not have a prefilter. The bleed off port will open after only a few uses, even though the dustbin has been emptied each time and never near full capacity. The problem is a layer of felted dust clogging the screen on the internal part of the canister. If I pop it open and brush away the dust layer with the vacuum's brush wand, it immediately works again, but only for about 2 uses. A Dyson support girl told me that dust clinging to that screen was due to static build-up inside the canister and that rubbing the internal components of the canister with a anti-static dryer sheet would solve the problem. But dust continues to collect on that screen. So I'm about to send it back for warranty repairs. :(

Two additional tips that may be helpful:

1. I used a dryer lint brush (long, small diameter, and somewhat flexible) to clean/clear several of the passages that were too deep or long to dislodge items that were potentially causing my Dyson to loose suction.

2. My filter had gotten a lot of buildup underneath the blue mesh top. I'm ordering a new filter but in the meantime I cut the mesh open, cleared the buildup, and stitched the mesh back together so I could use it.

Successful return to full vacuum suction.

Thanks for everyone's advice

I have a problem when using the wand on my DC-14. I’ve cleared/cleaned everything, but it appears to be the part that diverts air/suction. When I first turn it on with the wand free, for 1-2 seconds it sounds normal, but then the sound changes and there’s little suction from the wand. It’s find when using the floor/carpet part.

In that little vent on the canister lid beneath the handle, I see some sort of flap that looks like it’s not working properly (not sure if the vent is supposed to be open or closed when using the wand)—when the sound changes the flap is half open. I haven’t tried to unscrew that black plastic part that has a spring on it, which covers the part where the flap is.