An ongoing issue is that in recent times onboard staff have become used to the idea that ticket checking is a very low priority activity, either due to the social distancing necessary at the height of the pandemic or due to poor industrial relations causing low morale. While it can be expected that some people have sufficient self-motivation to find their own way back to previous standards of revenue protection for very many others it will take decent leadership from management. Unfortunately the currently increasing level of DfT intervention in TOC management seems unlikely to encourage such leadership.
It does of course go back before that anyway. When I started as a guard nearly all my work was pay trains. I regularly cleared £500-£1000 per shift (the most I took in one day was £1750).
At 3.5% commission I made a decent little extra bit of money and we still regularly sold long distance tickets, seasons etc.
The creeping in of TVMs e-tickets, smartcards etc has significantly reduced the amount that you take, although I still far more often than not take more than my wage for day, and often several times it. We did not ask for our routes to become Penalty Fare Zones.
This reduced the reward to the staff involved and the company has been quite happy to reduce their take home wage, whilst still wanting them to check the tickets, but leaving them to sell small cash fares and have a far greater proportion of time dealing with those who don't want to pay, rather than collecting fares on a pay train.
I personally am self motivated anyway, I like that side of the job, but for those who are now expected to accept more hassle for less pay whilst the company shrug and ignore the issue the result is obvious. I am happy to grind 200 plus quid in local fares on an evening train, others won't handle the hassle.
COVID also didn't help and nor does the perception that getting into arguments causes delays that are expensive.
It wouldn't take a management genius to come up with a comparatively cheap scheme to get people back into the habit of checking tickets, but I'm yet to see much of it.
Northern's pay to scan seems to work quite well - you regularly see the guards on their trains again now. Of course bean counters only look at it in pure quantitative terms and I bet even with scanning payments and commission in place, even with some refinement of the scheme to challenge misuse, they would pay out less to their guards than they did in commission 15 years ago.