Hello Joan,
It sounds to me awfully like the chord fingering is set to 'single fingered' which effectively means that you will get a full 3 note chord by simply pressing a single note (a root note) ...
So, to find out what settings your fingering is on we need to do the following:
Turn on the S900 and after it's initialised press the 'function' button on the display panel
> in the screen that opens select button 'C' on the left hand side of the screen
> then using the 'tab' button (top right of screen) tab to the 3rd screen 'Chord Fingering'
> once you have done this you will see in the bottom left of the display a 'Fingering Type' table you need to select 'Multi-Finger' by using the up or down button (#2) directly below the display
> then press 'exit' to clear the screen.
If you find that it is or was already set to 'Multi-Finger' ... we will need to look at other possibilities including doing a 'full factory' reset of the instrument ... not quite as horrifying as it sounds
I will leave it at that for the moment Joan, try this first and we can go through the steps of a reset if needed.
When you select your style and start playing a chord you will need to have the 'Acmp', 'OTS Link' & 'Auto Fill-In' selected with lights (red) on ... then select the 'sync-start' button and as soon as you play a three note chord in the left hand you'll be away
Lets hope that sorts it for you, if not get back to me and we'll try and work it out