Why running/walking on treadmill won't detect distance and steps. Treadmill records 6km but my Venu 2 Plus records only 1.6km. Steps too won't record correctly. For your info, I don't put my hands on the rail.

Does it count steps correctly when running/walking outdoors? Are you doing anything different with the watch inside versus outdoors? The step counter is unrelated to the activity and will count steps the same way. 

If the number of steps are not counted correctly indoors the distance will be incorrect. 

Let the arms moving naturally in order to have the steps counted correctly, Not only holding on the railing, but also having the hands motionless hanging may result in skipping some steps, if the treadmill dumps the foot impacts sufficiently.

And as for the distance - you can always re-calibrate the distance. You only need to run more than a mile or 1.5 km (shown on the watch, not on the treadmill) in order to have the Calibration available in the Activity menu after stopping. More details in the article: How Does the Treadmill Calibration Feature Work on My Garmin Watch? | Garmin Customer Support 

for few seconds/ 1-2 mins. Both hands were swinging like the normal running. Always on "motion" mode

The distance only started to move after 1.5km on treadmill. That's after I swiped the device screen many times. Same as the steps. I wore 2 watches on both wrists just to test and compare. Garmin (left) and Fitbit (right)

Fitbit has recorded almost 2k steps already but garmin not even 1 step. Until I swiped the garmin screen many times. Yes I will surely try again on my next run/walk. Hope it will be working good on all types of my workouts.

Lily said:

Fitbit has recorded almost 2k steps already but garmin not even 1 step.

If that is the case, I suggest contacting the Support and requesting a replacement of the watch.

Did you manually start the activity or did you just started to run and let the watch automatically detect that you were running? It is always better to start the activity manually.

Starting an Activity

I started manually