Another variable may be the kind of lead core line you have. I have the "no lead" lead core and I've noticed it does not go down as fast as the regular lead core.
The thermocline is starting to show up. Generally speaking leadcore sinks 4-5 feet per color. Like others have said it varies depending on a lot of factors. At the speeds you say you are trolling however I would say that your lead is sinking 4-5 feet per color. Right now with the thermocline showing up I would say you want to be fishing 7-10 colors out or set riggers between 32 and 42 feet.