Please let me apologise.. my comment was not meant to be pompous. Silenthandover has obviously never met me or he wouldn't make such a remark!
There are truly plenty of reasons for a non-alternation and the full stops after my posting were to hopefully lead to a posting from someone who might provide the answer.
Just for the record.. possibilities which spring to mind.. Strong SW wind can cause turbulence on one of the runways and this could delay the switch. Maybe there was important WIP in the undershoot? Maybe there was a crane in the undershoot? Maybe there was a lighting or ILS fault? Maybe there was some sort of special flight in the vicinity? Maybe a problem with some runway turn-offs? Maybe there was a broken light fitting reducing the length of the planned landing runway? Maybe.. maybe... C'mon guys; it's a 62-year old, long-retired brain dishing this out!