Siemens dw not filling with enough water

Hi I have got a problem Siemens sf64m330gb/44 dishwasher which has a large plastic water matrix on the side (the ones that hold a lot of water in preparation of the next fill). When the machine fills this matrix to the top, water passes through to the pressure chamber but as soon as the machine stops filling the matrix it doesn’t syphon all the water out of it. The matrix just remains full and I can’t work out why? The machine tries to wash with the small amount of water that gets through. The customer was told (when he bought flat) that this machine is 3 years old but has never been used, which I can believe because it is so clean but I suspect it must be a lot older has it has the old pressure chamber with 2 switches. Could something have gone due to lack of use? I would really appreciate some help on this, thanks in advance 🙂