Lots of "problem currently under investigation" excuses today on the Cotswold and Hereford route. Does this mean they simply don't want to tell the public what's going on, or that GWR▸ don't have a clue, either?
First indication of a problem on a signal panel - something odd. Could be a signalling fault. Could be a track fault. Could be an unusual but correctly indicated situation that's just starting to be indicated. Natural and correct action from signallers - "don't send anything that way until we know" and on a tight suburban setup that's cancellations and short runs even before the reason is know. "Something's not as it should be" is all they know.
Fast forward an hour or so ... the situation will be better known, but so much time and effort will have been put into getting the thing sorted, and to indicating which trains are running fully / short / cancelled that updating the "we don't know why" has slipped down the list of things to do. After all, what matters to people all the more is what is not running, rather than 100% accuracy and currentness of the reasons for the change.