servis dishwasher not rinsing well?

hi there everyone, and thanks in advance for any answers here. i recently bought a second hand servis dishwasher, and to begin with it was working perfectly, but after about 2 weeks it has started to not rinse the top shelf very well at all, sometimes it works ok, not perfect, but ok, but most times theres all the things on the top shelf are coverd in gross 'scaraming'! please could somneone let me know what i should be lookoing for in the way of causes of this, or is it time to get a new one :-( cheers everyone