Unable to separate blocks in my inventory.

So I play on a modded version of Minecraft, but I believe this is a base game issue as I see it is a slight problem for others that don't play on a modded version. The problem is that whenever I click a block or item in my inventory, it does not pick it up in the inventory and instead "shift-clicks" it into another interface or part of the inventory. I have tried everything, turning on and off touchscreen mode, forcefully crashing MC, and I don't know what to do. Luckily the game isn't unplayable as I have access to the recipe book, but I would really like to have the ability of crafting normally back.