Log in -> Open Prov or the Barracks -> choose an item -> click the far right end of the slider bar to make one full stack -> you may see something like this:
To sort it give the slider a wiggle or cancel that attempt and try again, if it's horribly stuck refreshing can help though it may still stick a little after that refresh. thankfully it only seems to do it with the first stacks after logging in Logging in, visiting other islands, refreshing and possibly other things can all cause this so keep an eye on it when placing orders.