DIAGNOSIS The 'Strong Brew' function on this model works by slowing down the water flow over the grounds, not by increasing temperature. If the showerhead is clogged or the water path is restricted by scale, it can't modulate the flow properly.
FIX Descale the machine (vinegar-water solution as described earlier). A clean, unrestricted water path is essential for the 'Strong Brew' cycle to actually slow the drip rate. After descaling, try it again—you should notice a longer brew time.
IF THAT DIDN'T WORK The button or control board itself might be faulty. Test by starting a regular brew, then pressing 'Strong Brew'. You should hear a slight change in the pump's sound (a slower pulse). If there's no audible change, the electronic switch for that function is likely dead.
HEADS UP Don't confuse 'Strong Brew' with hotter coffee. It just aims for more extraction from the same grounds. For noticeably stronger coffee, you're still better off just using more grounds in the filter.