Hi Mario,
You can use the guide for troubleshooting the YPG-225 provided by ifixit [http://www.ifixit.com/Wiki/Troubleshooting_Yamaha_YPG-225#Section_Keys_don_t_produce_sound|here]. There is a section labeled "Keys don't produce sound". If you need the manual for this keyboard, it is [http://www2.yamaha.co.jp/manual/pdf/emi/english/port/dgx220_en_om.pdf|here].
If those keys which do not produce a sound are somehow hanging (are lower than the other keys) and are somehow slack, the problem is related to broken keys. They need to be replaced. In this case, you can use [guide|10918|this guide here on ifixit].
Just keep us posted what you figure out.
Steffen