/clear bug

I've been making a map and I wanted to use the /clear command. I used a simple comparator-clock to activate the commandblock with the /clear command. I want, that everyone, who is in a specific area, get cleared his inventory, but somehow it works not everytime

The exact command:

/clear @a[x=71,y=84,z=-7,dx=,dy=1,dz=5]

I am in the right area, I've tested it many times.

In the first screenshot, you see the clock and the commandblock with the command. In the second screenshot you see me in the area; you see how I am holding a quartzblock. That means, my inventory doesn't got cleared.

It works, but somehow not everytime. It isn't lagg, because the redstones seems to run smooth.