Trading In Champ Select

What determines with who and how many people you can trade with in champ select? I've asked this before and got no answer. Im wondering because I had a champ select the other day where i was going to trade champs with someone...but after picks niether of us had the option to trade with the other. I was 5th pick and he was 1st and my only option was to trade with our 4th pick. He said he did not have the option to initiate the trade with me so we were both stuck playing who we picked.

But then there are other times like my last game were i had 4 options, i could of traded with all on my team.