Probably a really simple explanation. Have you ran a deep clean in the last 6 weeks - 3 months? The citric acid solution it recommends you use prevents built up of limescale etc.

Give this a read: should see you fix it.

Humidifier Question - Water level depleting slowly | Dyson Community

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Check the base cap at the bottom of your pump for a blockage - This is the small cap is at the base of the pump and can easily be removed. 

Try the deep clean after - Running a Deep clean cycle | Dyson Community

Give this a read: should see you fix it.

Humidifier Question - Water level depleting slowly | Dyson Community

“”

Check the base cap at the bottom of your pump for a blockage - This is the small cap is at the base of the pump and can easily be removed. 

Try the deep clean after - Running a Deep clean cycle | Dyson Community

 

I took off the screen and cleaned and tried to run the clean cycle.  I can’t get through the clean cycle though because the machine thinks the tank is empty and won’t let me proceed to starting the actual cycle. 

This issue is not that it is working slowly or inefficiently, the issue is that the water level sensor is not functioning.  It is telling the machine that there is no water, so the pump doesn’t even try to work.  
 

The pump and evaporator are bathing in the citric acid solution now, but I am unable to run the clean cycle because the sensor is telling the machine that the tank is empty. 

If the sensors broken/not working, you’ll have to buy a replacement. I’m wouldn’t be sure if this lives in the pump or the machine. Either way, Dyson may offer it free, as you’re only just outside the warranty and you’ve had the same problem before. Have you tried calling them? 

What do the 'Alerts' mean on my Purifier Humidifier LCD screen? | Dyson Community points towards their online help for a solution. Might be an ideas to start there. 

Tank Missing | Troubleshooting Steps | Dyson UK

I have exactly same issue, and as my machine is out of warranty I was redirected to the paid service. This is disgusting that such brand has so serious issues. My first humidifier failed on the first run and second during 3 year of operation. It would be great to receive extended explanation what needs to be checked. All those advices posted here are not working, as they were not for the first author of this post. Help needed!

I have the same issue.  i bought a new water tank and pump and still it wont work.  $150 later.. and the customer service team just keeps repeating the same thing over and over.  They now want me to box up the unit and ship it to Calgary, I am in BC to see if they can fix it. 

If any of you know how to fix this issue before I spend another $200 + to get it fixed and ship.  Please let me know.

Kinda a waste of $1100 on this unit if it only works for a few years then nothing can be done to fix it.