Only getting nibbles is not necissarily due to the bait.
When I was lower rank, and my rod was getting under 50% I would always get nibble first, with a possible nibble on the second wait if it was black sole.
I would have to suggest only getting nibbles from this fish is a function of your rank and or stats being to low.
To the degree that at a low enough rank, it is not even possible to have high enough stats to not get nibbles.
Even before the SP changes, my opinion was that if you were of rank to fish black sole efficiently, would be sacrificing SP, because you could be easily fishing in grade 4/5 waters by that point. Black Sole is a grade 4 type fished, based on it's rank, that got stuck in grade 3 water, due to the geographical information provided by the devs, since there is no grade 4 salt water in the Uldah areas.
Once everyone is 50, and people are fishing for mats/profit, this will be the target fish.
When SP is the primary consideration, this fish is a major wrench in the system if you are trying to also fish up usefull materials while grinding.