I have 8 lower keys on my piano that sound dead?

I have a few keys that don't ring out. All the other keys sound riiiiiiiiiiiiiiiiiiiiiiiiiiiiiing but these eight sound silenced. You can hear the note, but it stops dead as if someone was holding their finger on the strings inside. It's an upright and all 8 keys are in succession. Could there be something down there resting on the strings?