How can I get the hard top off my mercerdes SL280?

SL 280 convertible, 129 series, 1994
I have tried to remove the hard top of my merc but the button does not unlock the front of the roof to enable me to take the hard top off. I know I can do this manually however, I think it maybe because the hydraulic fluid may have leaked, if this is the case can I and where would I top up the fluid? Or could there be another reason?