Why Ventolin out of stock

I get my chronic medication from Dischem because they are SUPPOSED to be the best. For the last 3 months they have had no stock of Ventolin for my asthma. I went online to Springbok Pharmacy every month for those 3 months and within a day they delivered Ventolin to my doorstep cheaper than Dischem ever charged. Why do they have stock and Dischem doesn't. I know who I will be changing to in the future.