Good info. Test your bait to make sure it's hooked right, has the right action and see how fast it falls. I think the Senko is something like a half foot per second or so. Also depends on the hook and the rig. Line diameter.. line type.. etc.
Monofiliment floats, so will make your senko sink slower than fluorocarbon and so on.
To make it fall faster, you can "nail weight" it. By either purchasing nail weights, or clipping the shank off some small finishing nails and inserting it into the worm. This usually isn't necessary unless fishing deeper water than 10 feet though. It has a good fall rate to draw strikes on the fall as it is. They actually make lighter senkos that fall slower too.