Hi, this looks like a 'cold shut', I have no experiance with lost foam, most of what I do is based on traditional silicate bonded molds, it looks like either:
1) Too much gas is being liberated by the foam in the fins, stopping them from filling, liberated gas may be preventing the alluminium from being able to burn out the foam.
2) There is not enough heat capacity in the alluminium that has started to fill the fins to complete the burn out.
Solutions, maybe.
a) Make the main web heavier, this will feed heat into the fin area helping the burnout, machine off the unwanted thickness afterwards.
b) Precoat the foam pattern with a silica based dip, similar to lost wax casting and burn out the pattern in the furnace before pouring, setup in soft sand and pour.
David
( never stop learning )
http://www.steamcastings.co.uk/