Hi,
The +/- is working perfectly.
The problem comes from your template/design where the input box for the quantity is too "small", the text into is not visible because more width is needed. You should add some CSS override in order to change the way that this input box is displayed.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.