did you purge the steam tip soon-enough after you last used it? I have no experience with this machine, but if its steaming into the drip tray it may be blocked up? Although my Breville dumps a lot into the drip tray after steaming so it could just be a cooling flush (does the 5900 have 1 or 2 thermoblocks?)
I dont mean to state the obvious, but its worth asking! Can you remove the tip or use a pin to see if its blocked?