There are tons of clarifying agents out there, keiselsol, chitosan, sparkloid, isinglass, the list goes on, lots for you to try.
Try searching for "fining" in the title of threads, that should get you plenty to read for a while.
Also, with the bentonite - 2 questions. Firstly, was the bigger batch that you failed to clear something different than what you'd ever cleared before? Might not be the fining agent's fault, some particles are positively charged, some negatively, bentonite can only grab one of them (can't remember which honestly). Question #2, how much bentonite do you use in 1 gallon batches, and how much did you use in the bigger batch?
My personal method for clearing mead is time. I wait. Then I wait more. Then more. Then the mead is perfectly clear, and well aged.