I'm no catfish expert so take this for what it's worth. I'm sure much more knowledgable folks than me will chime in but here's my 2 cents worth.
First, I would be looking at Tawakoni rather than Fork. Nothing against Fork, it's a great lake. But, Tawakoni has large numbers of Blue Cats, which grow quite a bit larger than channels. I'm sure there are some Blues in Fork but I think they are few and far between.
Second, as said above you need to be using big fish bait. I'm not sure what you've been using but if it's been some kind of prepared bait (punch bait, dip bait, dough bait) then you're going to catch mostly smaller fish. You might get a big bite on it but, again, those will be few and far between. So you need some kind of fresh cut bait (shad, bluegill, carp and the like).
Third, not sure if you're in a boat but I'm assuming that you are. Find a wind blown point and concentrate on that. Put several baits out at different depths until you find the depth where the fish are at.
Lastly, this isn't really the best time of year for catching the really big catfish. Colder months are typically better. Not saying they can't be caught this time of year. And if I were chasing the big ones I'd be using the above approach. But like I said, I'm not an expert and I didn't stay at a Holiday Inn Express last night.
"Life is a matter of luck and the odds of success are in no way enhanced by extreme caution." - Erich Topp, German U-boat Commander, 1943.
When in doubt, set the hook.