Very similar to starting fish to eat pellets by using freeze dried krill-it's not fish meal. I have a hell of a time getting fish to move from krill meal base to fish meal. Same thing is happening to you.
I have found no tried and true method to get them on fish meal other than just quitting the other feed for an extended period-we are really messing with mother nature here.
We can feed krill based starters but if we have to keep them on the largest premium starter, it's going to cost big money to continue that long term.
I'm told the krill has a flavor or "taste" very similar to zoo plankton, so there is a part of the puzzle. How do we get the fish to transition to fishmeal ? Time and effort is all I know that will work.