840 with Favero Assioma Pro RS Pedals

Do you pair it via ANT+ to the Edge? check the connection type in the sensor details in the Edge. And probably you need to toggle on the "Cycling Dynamics" there.

check with Assioma app , in the settings, is there any option to switch on/off ANT+ and is it not switched off ? usually there is such an option in powermeters' apps. ( i dont have Favero stuffs so i cant tell it)

I'm also using Assioma Pro RS, the dual sided version. I only get Power Smoothness and Torque Effectiveness displayed on the Live tab of the Favero Assioma app, not on my Edge 840. I also don't get any values for these two in the FIT activity files either. Not sure about force metrics. Being available on the app means that they are being transmitted over Bluetooth, but when I pair the Edge using Bluetooth connection I get even less fields displayed and recorded (e.g. no Pedal Phase or Platform Center Offset). My guess is that the ones you're looking for are not transmitted over ANT+, at least not on the Garmin-compatible Cycling Dynamics way.

via ANT+ and Garmin:  the BT protocol on Garmin is accept only power and balance and cadence, nothing more, so only the ANT+ connection will show you these cycling dynamics data. you need to wake up the pedals, go in the sensor menu in the Edge, details, and there there will be a cycling dynamics toggle (because if it off, the Assioma is working only on 4Hz, with on, 8Hz with CD datas). the Favero App via BT is using an own protocol, not the standard one (which is implemented in the Garmins)

as I know the Assioma is not transmitting raw Force values , only a few powermeters can (with additional setting in their own apps), like Power2Max NG series.

sensor menu, there is the Cycling Dynamics submenu (yes, i know it is a 1050, but in the 840 is the same):

and there is the toggle for CD and for the TE and PS , both must be ON

I found how to display Pedal Smoothness and Torque Effect:  I don't think the Edge supports "Force".

Your Edge must be connected to your pedals as if going for a ride, then:

From the menu select Sensors for your Profile.

Select your Power Sensor(s) (Favero)

Select Sensor Details

Select Cycling Dynamics

Switch Cycling Dynamics to ON

Switch Torque Effect Pedal Smth to ON

Assuming you have Torque Effect & Pedal Smth Data Fields in your power Data Screen you should now see the metrics.

May need to do the above for multiple Profiles if applicable.

Correction: I don't think Favero supports ""Force".

i wrote the same above (about enabling the TE/PS and about the Force ) :) 

and the Favero NOW doesnt support the Force, but i assume it will do it soon with a firmware upgrade, the Garmin will release (rumour says...) a new Rally 2.0 , with new IAV power calculation method (they used AAV method till now which is not the most accurate with every pedaling style) and rechargable battery, with Force -support