On promotion but no stock

Hello <br> <br> I went specially to Incredible connection , glen mall , to buy mouse, for my computer which was on promotion <br> <br> I was working whole week, could only go on sunday , when i went on sunday , i was dissapointed to hear, mouse was out of stock <br> <br> How come items which are on promotion, always run out <br> <br> <br> Can anyone please assist<br> <br> <br> <br> <br> <br>