@Rizzo - I don't know the specifics of your keyboard, but for any digital keyboard that tries to emulate an acoustic piano, there are two possible routes:
1. Sampling (recordings of actual piano notes, nowadays multiple recordings per note, at different velocities).
2. Physical modeling (software that builds the sound up from an algorithm which "understands" hammers, felt, strings, etc).
Some keyboards only use sampling (and my guess would be the older the keyboard, the more likely this is the case).
Some keyboards only use modeling (Roland V-Piano).
Some keyboards use both. For example the Numa XPiano uses sampling plus modeling for acoustic pianos (this is how they are able to offer you control over how much string resonance you want), and modeling alone for electric pianos.
My simplistic understanding of string resonance (based on over 50 years of playing real pianos, and almost 20 years of using digital pianos) is that it is the result of the complex interactions among all the strings that are currently "undamped" (free to vibrate), whether they are being struck or not.
Sampling alone (playing and recording one note at a time) obviously cannot capture any of these interactions.
I *think* you need physical modeling to have string resonance.
