Samsung GT N7000 default ringtone

Until last week, ringtones set for specific contacts worked perfectly. For no apparent reason,
the default ringtone rings when these contacts call me, in spite of the fact that the individual
ringtones original assigned to these contacts are still in place in their contact details. Also, the
contacts are stored on my phone and not on the sim. Can somebody give me some advice, please?
There is no update for my phone available. The software is still as it used to be. Thank you so much.
:(