Welcome! My 2e ds has a relatively low WM score when he was first tested with the WISC (7 years old), but he also had a PSI lower than the WM. Working Memory refers to the memory available for use as you're working a problem (sorry that's not a very technical answer!). A low WM score can be a concern if there are other issues causing an overload on WM, but in general I wouldn't be too concerned about the score. When we asked our neuropsych about ds' WM score when he was 7 she said that her view of it is WM is either "on or off" and a WM lower than PRI/VIQ etc isn't a cause for concern in and of itself. She also mentioned that WM sometimes goes up with age, and that absolutely happened for our ds - when he was re-eval'd prior to going into middle school, his WM score increased significantly.
Best wishes,
polarbear
