I may regret saying this but, I think you guys expect too many assists. Why does it matter anyway? Your team got the kill, move on. I have no problem getting assists and am nearly to Onyx with assists. I thought that meant I sucked but I've found I get the most when doing my part and playing as a team.
Here's a recent round where I went +8 and also got 8 assists.
That said, I wouldn't be against a slight increase in the time window to get one, and the way assist challenges make people play is dumb just like the jet pack challenges. What I do is tend to ignore the challenge on these days and usually ending up getting more kills than normal which is fine with me.