Hmmm this one is kind of hard to describe but i will give it a go... 

I am at the vendor at 20,24 and are purchasing drinks... i can beers and 6 packs of water fine but are having problems when i go to purchase large waters and small waters.  I buy the drink... the money gets taken from my account... i have them in my items... then when i go to buy the other type... say the large waters... it deletes all the small waters i just brought from my backpack... i have 20 backpack slots so i am not sure if that has something to do with it... 

i have tried moving spots... it also did it to me in the one next door... i have tried moving off the vendor location then going back... it still did it... and i have tried using a drink... it still did it... 

Hope that makes sense...

hmmm further to above... i think it maybe a backpack problems as it is happening randomly... like now when i move around drinks go missing from my bag feels like my handbag in real life...

all the items in the bottom row of my backpack have seem to disappeared a few times...i think this kinda what happened here

same happen 2 me.. i was buying six pack waters to take back then bought a large water because i can't use the sixpack because i forgot to put the fridge in my cell then the sixpacks i just bought disappeared

been testing the back pack now, i filled it refreshed the page a few times and it threw out 2 quest items..

yes it is a little frustrating lol...

is it a bit like what these peolpe have had?

http://www.prisonblock.com/forums/Items-not-going-to-backpack-t34307.html

hmmm it isn't about not pickign them up... it is about them going once they are in there...

people thought i was joking about losing a clown wig

people on that post have had things taken from their last slots in their backpack and placed in inventory, check there, just a suggestion