If na.rm=TRUE (the default) then elements of a variable that
  are missing (NA) for any iteration in at least one chain will
  be dropped.
This argument was added to handle incompletely defined variables.
  From JAGS version 4.0.0, users may monitor variables that are not
  completely defined in the BUGS language description of the model,
  e.g. if y[i] is defined in a for loop starting from
  i=3 then y[1], y[2] are not defined. The user may still
  monitor variable y and the monitored values corresponding to
  y[1], y[2] will have value NA for all iterations in all
  chains. Most of the functions in the coda package cannot handle
  missing values so these variables are dropped by default.