I can't get this unit to secure to the bars. Is there some trick to it?

Mod. HL.EL.120

You pull the rubber strap until it is tight and then flip the little plastic clamp down. The clamp locks the strap in place and tightens it a little.

Tried that - doesn't work. The clamp doesn't have enough tension in it to hold the strap. I prefer the older type that latch on positively.

I think know what you mean. In an attempt to make mine more secure, I pulled the strap really tight and pushed down the clamp so hard I broke half of the plastic clamp off.

Did you push the clamp all the way down until it stops against the strap? It's definelty hard to make it stay in place. I got mine to work but it moved around on a ride. I had two of them. I finally gave the good one (without the broken clamp) away and went to something else.