[Chimera-users] discontinuous ribbon for nuc. structure

Hello, While i make the ribbon representation for an RNA duplex, sometimes ribbon is discontinuous in between, what could be the reason for it ? I hve two conformation sof the same duplex taken at 1ns interval from MD trajectory. For one conformation, ribbon is continuous but for the other there is a discontinuity at the middle.. I analysed the structure, it seems ok. thanks in advance, Bala