If the streaks you are leaving are where you overlap, these are normal, as far as I can tell. If you think about it, you are cleaning those striped areas twice, so the stripe is actually just cleaner than the surface between them.
There are several things to do: Adjust the bar higher; If the surface is mildewed, downstream bleach onto it. I find that adding the bleach after the cleaning works well; If the surface has sprinkler rust, I spray the concrete with oxalic acid (6oz/gal).
On some driveways with rust/heavy mildew, you will need to lift the front end as you turn the Water Jet. I follow the "grain" of the concrete to minimize the streaks.
As to the spray arms possibly being bent, if they are, you will probably feel a vibration. My Water Jet had a bad shaft that wobbled, so it vibrated all the time. (I bought mine used, so I thought it was normal!) It seemed like every week I was bending/breaking arms, till I had the shaft replaced. It now runs as smooth as silk. What a difference.
