question marks in linear regression output

I ran a linear regression model with 18 independent variables and feature selection turned off.  For some of the independent variables there were question marks for the standard error of the estimate, and therefore for the t-statistic and p-value for the coefficient.  I ran the mode again with feature selection turned on and got the same question marks.  What do these question marks mean?  Thay cannot have anything to do with missing values as the regression would not have run to completion in that case.  I am baffled about what these "?" symbols might mean.  Help.....