Hi
a----the html syntax is good : the label of the button is define by the attribut name value= " xxxx".
you have to have look to the css properties (attribut name /value) of the class selectors name .btn .button .hikashop_cart_input_button.
parent class could also explain the issue so best is to give an url to study .
b----which premade template do you use ?
it seem you use bootstrap 2 ( span selector ? ) but many template use it
c----dig around padding margin , size ,blank ,transparent, hidden , not display text ! ( not button )
regard's