The size of the lead needed to get to the bottom varies with the conditions and line thickness. A heavier line is thicker, creating more drag in the current. We prefer a lead heavy enough so we know it has reached the bottom. Fishing in a strong current and not knowing if the bait is on the bottom is a real pain and generally not very productive. Sometimes you'll find you can reach the bottom but can not hold it.
Water pressure creates a belly in the line that lifts the bait out of the strike zone.
The way to capitalise on this effect is to wind in until you're reasonably sure the belly is out of the line. Five to ten winds will generally do, then free spool the lead down until it hits the bottom. This can be repeated four or five times or until too much line is out. This system of walking or drifting the bait along the bottom covers a lot of ground and can be extremely effective. Don't expect to get it right the first time because it's not easy to master.