On previous flights using an iPhone 6 the airfield plate has appeared automatically when commencing taxi and again on landing when turning off the runway. On the last two flights this facility hasn't activated and I am at a loss to understand why.
I imagine the operation is trigged by location and speed ?

Yes, but you do need to have the plate loaded (cached) as well. Which airfield?

I hadn't knowingly done anything to load or cache the plate for EGNR it just appeared automatically.

The only action I have performed recently was to upload the revised UK chart as advised.

Unless you have viewed the plate at some point, you won't actually have it on your machine, so SkyDemon can't load it automatically when you're in Go Flying mode. It's as simple as double-clicking on it to view it, then its background will turn green which means you have it on your machine.

Thank you . I will try that.
Should the airfield plate then automatically appear on first movement of the aircraft off the apron and on landing back at same airfield.
It did this previously after first moving off the apron , is this taxy speed dependent or geoposition or both ?

I can't remember the exact parameters, and they are not defined as they're subject to change. Certainly when you land it should zoom in and show the plate, and also when you initially press Go Flying when you're on the ground.

I viewed the airfield plate and observed the selection turned green but when I tried this on Saturday the plate did not activate either on initial taxy or following landing.
This feature worked previously and I am just curious as to why it doesn't work anymore.
Can you please advise if there are any other settings I should check to make this operational.
I tried the iPhone 6 that I use on both its own "location service" and on a "Flarm" connection.
Many Thanks

It's probably worth contacting our support about this. They will be able to talk through it with you, identify exactly what you're doing and what should happen.