Every operator of vending machines needs to be sticky about replenishing the stocks regularly. Particularly if your vending machines are the combo variety, then the chances are that they don't hold much of any product. Some products have a very swift offtake, whereas there maybe few takers for some other products like health foods in college. However, if you keep separate vending machines for dispensing snacks and drinks, then you also need to keep track of the expiry dates of products like crisps and confectionary and replace them accordingly.
The biggest advantage of vending machines is that they are usually self-service. This turns into its disadvantage if you don't keep track of the offtake of the products you have on offer. Suppose somebody likes to buy chocolates from your vending machine. If s/he finds it out of stock even two days running, you will probably lose your customer.
This means that you need to look slippy about replenishing the stocks of your vending machines. You may have fixed specific days on which you go round restocking your vending machines. However, it makes very good sense to have your mobile number printed where most customers can read it so that they can contact you in case of difficulties. This would be useful not only to give you an update on the level of stocks but also in case of any malfunction such as the vending machine not returning change. If any of them are located in a shop or similar outlet, then you could leave your number with the manager or owner to give you regular updates.
By the way, from where do you normally buy your vending machines? Do you source them directly from the manufacturer? Or do you source them from people advertising vending machines for sale or sites like Craigslist and eBay? When you opt for buying vending machines for sale you should not buy them blind; i.e. you should see each piece before purchase. And make a point of finding out why the vending machines are up for sale . Is it because the vendor is selling his business or because the venue is unprofitable or is there something seriously wrong with the vending machines for sale?