My CC 3-6" went into hiding for 7 months after I stocked them. I figured they all died and sunk...until one day they showed up and have been fixtures ever since.
Take home message, give them a few months.
Bingo. Same here. I can remember when we first started seeing 9" or so long CC sneaking up towards the edge of the pond and stealing a pellet that had sunk then disappearing. My first stocked CC were extremely shy the first year. Did not see them for a long time after stocking.
You could get some feed to the bottom in a couple feet of water. They sell sinking catfish food but not many places carry it (one local coop in our area does). You could also pre-soak some floating feed so it would sink or do as was previously said throw it in a feed ring to contain it and it will sink after several minutes.
They will find it. You might not see them but they will find it. If you see muddy water riled up where you put the sinking feed something is eating it. Likely the CC. The CC will be cruising the bottom looking for something to eat and when one fish finds the food the others in reasonable proximity can sense the feeding and will show up.