There is a chance you just got single mode one.
The C8 comes in a variety of configurations so maybe your one has a single mode driver instead of a 5 mode one.
The modes change each time the current is stopped, which is what lightly pressing the switch does.
AT YOUR OWN RISK: You can imitate this by taking the cap off, leaving the battery in and connecting a thick wire from the threads (or some other unpainted part there like the edge of the battery barrel) of the body to the battery’s –ve. Every time you quickly disconnect and re-connect the wire, completing the circuit it should change mode. So you can see if you have five modes and the switch is just broken.
If the switch is broken you can clean it or ask for a replacement etc.
Some torches will switch to a lower mode as the battery gets depleted so if the battery is nearly flat you may only be able to use one mode, though the flashing modes should still work. I also do not think the C8 has this sophisticated capability. However, try it again with a full battery (note that the Ultrafire batteries may have quality problems of their own. Make sure, assuming you are using a cheap charger but suitable for 18650, that you measure the voltage of the battery every hour or so, making sure they do not get overcharged (about 4.2V should be the max voltage)).
Really, your best bet is to ask for a replacement or discount on the product you got as it is either a different product to the one you ordered or broken.
Some of the experts here should be able to help you soon.