I think pappy nailed it. It might have been more to do with depth and less to do with speed. Simply put, your drop down was too deep for that slower speed. The same drop down had more lay back in the water at a higher speed thus it put the bait higher in the water column.

For what it is worth, I've had my best success trolling at .5 to .8mph. Several years ago, I was fishing in a pretty heavy wind. After fighting and fighting it, I decided to just let the wind push me. I took note of my speed while this was happening. It didn't matter what depth I was at and the fishing was fast and hard. Since that time, I've mimicked that speed on several occasions with the same or similar results.

I will say though that this is long lining. I do believe there is a big difference. A couple of weeks ago we had an outing on a local lake of mine. One of our members was pushing jigs (spider rigging) while I was long lining. We were running similar depth and speed. He caught 2 and I caught 22 fish.

For me, I adjust my depth by the weight of the jig. 1/64, 1/32, 1/16, 1/8 and even 1/4 oz jigs. Last weekend, my buddy and I did this with 4 rods. Each of us ran a 1/16oz and a 1/8 and both rods caught fish.