Maybe it was a question of beginner`s luck but my Mavic Air responded to my hand gestures the first time that I tried it.  However I had a difficult time getting it to land.  Now however I cannot get the Air to recognise my hand at all.
Any help will be appreciated.

Can you post a video of the 'error' happening.

I had the same issue particularly when trying to take off and land. Landing took awhile, and for a minute I thought I may have to go back to my phone, but after quite a few tries it finally landed. When the drone was in the air it was ok.

Please refer to the tutorial below and make sure the steps are all correct. For the palm landing, you need to move your palm slowly to make the drone descend. If this doesn't work, please provide me with an operation video for further checking.

DJI Thor Posted at 2018-3-25 19:04
Please refer to the tutorial below and make sure the steps are all correct. For the palm landing, you need to move your palm slowly to make the drone descend. If this doesn't work, please provide me with an operation video for further checking. https://www.youtube.com/watch?v=mueLjVXujP4&t=0s&index=8&list=PL8-vxWY64sBOeP6CJM0vmkDB0onco2V2i

Thank you for your reply.

As soon as weather permits I will make a video of the whole process and send it on.

I can imagine that low light/contrast environments might be more difficult for the AC, fwiw.

Anything change between when Mavic Air did respond to hand gestures and no longer does?  
Like update to Firmware?  
You were wearing gloves before or now are?  
Bright daylight before, since cloudy?  
Possibly change in flying locations, thus different backgrounds?

I have had reasonable luck. Seemed more challenging getting it to land, but it finally worked. I have, tho, experienced that lighting & contrast can have a big effect - both too little and too much. Once, doing the jedi palm thing and sweeping around horizontally, at one point there was too much sun backlighting and it lost me. I've also had that happen walking around in Active Track as well when my movements resulted in big lighting / contrast changes causing it to lose track of me.

HendrikJB Posted at 2018-3-27 11:47
Thank you for your reply.

As soon as weather permits I will make a video of the whole process and send it on.

Looking forward to your update.

DJI Thor Posted at 2018-3-28 02:50
Looking forward to your update.

Good day folks.

An update.

Today I tried it again and found that maybe I was a bit too quick with the hands.  It seems that one really has to be slow (though the videos I saw did not look extra slow to me) with the hand signals.  Specifically the "down" to force landing.

As I have not yet specifically tried the contrasts that were mentioned it does seem as if one has to be very slow with the hand for landing.

HendrikJB Posted at 2018-4-2 11:31
Good day folks.

An update.

Thank you for your update and glad that you found the way to work. As for the gesture speed, the pilot needs to slow the motion to let the drone recognize the order then execute the order.
You can try to test more.

I managed to take off with hand gesture but that was it. In the end, I had to grab it and turn sideways to switch off. But I will persevere next time I have time.

HendrikJB Posted at 2018-4-2 11:31
Good day folks.

An update.

Glad you've fixed it!

A CW Posted at 2018-3-23 10:31
Can you post a video of the 'error' happening.

In the end it just seems to be a problem with the pilot as I get it to work the way I want to in about 99% of the time

HendrikJB Posted at 2018-4-17 10:29
In the end it just seems to be a problem with the pilot as I get it to work the way I want to in about 99% of the time

Happens - just need to take your time I guess.