I have no problem taking a relay from the enemy's relay post, but can't return it to our relay post. So far I've tried:
1. "touching" our relay post while holding the enemy relay,
2. droping (using F key) the enemy relay onto out relay post,
and both had no effects: the relay just remains in my hands / on the ground.
Am I missing somthing here?

Your teams relay must be returned to your base first. Sounds like the enemy had your relay when you tried to capture. All you have to ruins run up to the capture point in your base while holding the enemies relay.

I've also tried and couldn't do it

Your teams relay must be returned to your base first. Sounds like the enemy had your relay when you tried to capture. All you have to ruins run up to the capture point in your base while holding the enemies relay.
Thanks
I knew I missed something~~