Hopper doesnt work with custom block

Im making a block with can turn one item into another. id like to autimate this process with hoppers. but when i connect a hopper to the block the items diassapeat into the block and dont show up in the input slot. it seems to have a 3rd invisible slot because after one stack goes through the hopper, the items will start to go into the input slot. this happens wih other blocks to where i didnt really change any settings. does anybody know how to fix this?