I find my ability to drag/move inventory items depends on the item, the type of inventory (personal or household), the number of items in the stack – and whether an item(s) is 'stuck'/glitching.
For instance, I can drag stacks of produce items into the Lice Cold Freezers as a stack, even if the stack has over 99. BUT, if I want to sell produce in a stack, USUALLY if the stack has over 99 I need to open the stack and sell them one by one until I get to 98. Then I can sell the stack.
Also... I've had situations where some of the items were 'stuck'/bugging. For instance, I had flowers in a personal inventory that I was trying to add to the flower arranging table's inventory. I couldn't move the stack for one of the types of flowers. I could move them if I opened the stack and moved them one at a time (but not from the top of the stack). I found that several of the top of the stack flowers would not drag, were not delete-able, and were 'stuck – plus they were making the stack 'stuck' when trying to drag it as a whole. However, if I select the 'stuck' item(s) one at a time, I CAN request that the sim 'place in world'. They will then place the item(s) on the ground. If they're not on your lot you can't do anything with the item, but if they're on your lot, you can then drag them from the floor to delete or to sell. If you fix those 'stuck' items first, then you can move the rest of the stack as a whole (unless 99 or more). Usually.
It's a bit funky and I've had to creatively use these methods in a number of situations where inventory items are 'stuck'/glitching, but it works for me.