Bosch SMS50C26UK Dishwasher not using salt and not cleaning

Hello,

I have a Bosch SMS50C26UK/B3 Dishwasher that is not cleaning very well, it never uses any salt.
The salt container contains water and salt but the salt is not used.
Cups are always tea stained.

Any help as what to look for.
I am good at fault finding but dont want to take too much to pieces if there is anything obvious I should be looking for.

The water jets on the arms are clear and it makes all the right noises and drains well, just doesn’t clean very well hence the suspicion of salt.

The dishwasher was bought in October 2015 so about 2 1/2 years old.

Thanks.