Bowflex M5i - how to increase resistance? Upper body exercise

Hi all, 

Any help on this would be much appreciated. 

I bought my Bowflex 3 years aho second hand. When I bought it the previous owner must have had some presets as the arm resistance once was quite strong meaning that I had a good upper body exercise as well as lower body.  

I wonder if I've inadvertently changed something as now, even when I set it to the highest resistence setting (16) I hardly feel any resistance so that my legs are spinning around really quickly and there is almost no point using my arms as there is almost no resiistance. 

Have I changed a setting unintenionally? What can i do to increase the resistance again please? I've had it three years and so have got used to the machine, potentially making it easier for me, but this doesn't explain the drop off in difficulty. 

In general does anyone have any suggestions about how to make it more challenging? Particularly for the arms/upper body? 
 Any help would be very much appreciated.