Users are unable to perform RX Margin analysis on the upper channels of an IBERT design when the channel numbers are not sequential.
For example, in a design using channels 113, 114, 117, 118, and 119, only channels 113 and 114 could be analyzed. The upper channels will fail attempted analysis. If the first two channels had instead been numbers 115 and 116, then this issue would not appear.
This is currently a known issue, and it is set to be fixed with the 14.4 release of the tools.