Unfortunately, Xref is optional under NOV so many systems do not automatically provide it even though it is one of the recommended headers. To summarize, make sure that your server provides the Xref header AND if it supports NOV, make sure that the Xref header is part of the NOV database. Many key-bindings accept prefix arguments. A prefix argument is an integer that is generated prior to a key sequence and is used as a way of controlling the behavior the key sequence. It is generated by pressing the ESC key and then pressing the keys that correspond to the integer.
Some functions use such arguments as repeat factors.
That is, the function bound to the key sequence that they modify will be repeated the number of times specified by the prefix argument. Some other functions may simply use it as a way of slightly modifying the behavior of the function. Without a prefix argument, the headers will not be included in followups. This means, press ESC and then a number followed by the key sequence that you use to enter the newsgroup. If slrn is run in an Xterm, it can use the mouse.