UPDATE: Hi Guys, I opened up the shifter mechanism and it turns out that it is OK and not broken.

Something more significant might be, the Gear Hub at the back might now be faulty. I am getting the Highest, and the one below, but not the third and final (i.e. lowest).

Can these be fixed? if not, are they expensive to buy? I look on SJS, but I don't know what the back Gear Hub is called, there are so many tick boxes :/