Yeah, sometimes if you have a stack of a certain provision, when you then try to add one or more copies of it to a ganger's backpack the game will just refuse to do what you want. It's a bit weird like that - you will hear the correct sound effect, but nothing will happen
The workaround is to go to the store, sell every copy of the stack except for one, then put that single remaining copy into the ganger's backpack. Then go back to the store and buy back the rest of the stack. If you have more copies of the stack that you want to add to other backpacks, only buy the provisions back one at a time.
This is one of many odd little bugs in the game that has a workaround, you just have to think creatively to figure out how to outsmart the game's stubborness. XD