Mike

In the light air it would appear that the flow is breaking away from the sail somewhere between the middle tell tails and the leach, this is not a major problem since the drive comes from the front 1/3 of the sail, but there would be less drag if you could get them flowing, (flowing for 75% of the time would be a good result,50% perfectly acceptable)

The reason for the loss of flow may be that the draft on the sail is along way back, or it could simply be too full.

Either way more downhaul will pull the draft forwards and flatten the sail.

In very light winds pull on a lot of downhaul, in fact the same amount as for 20mph +, (in less than 5 mph most sails are too full and the air will just not stick around the big curve the sail is asking it to and also there is much more wind shear so the top of the sail has the apparant wind from a free-er direction),this will help the leach ribons flow in these conditions and is faster but in 5 - 10 mph you should be using no downhaul really, but maybe it's an old sail that has gone very full, Try more downhaul, (you will get the ribons to flow)but you may find that the loss in drag doesn't make up for the loss in power.

In the stronger winds there is no reason why the top leeward telltail shouldn't flow, It's well above the jib isn't it so jib sheeting can't effect it, derotating the mast will de-power the rig, and you should go faster in the higher winds, but it shouldnt affect flow.

Is there anything on the mast that may interupt the flow (it might be that the spinnaker halyard is blowing around just in front of this telltail.