In my observation it's south of Northampton where there's an issue primarily - and it is a fairly big issue - there are maybe a handful of guards you see doing revenue, and you recognise them - the rest hide in the cab. And one of those handful (who I know personally) is no longer a guard due to medical reasons and is now station staff, so now there are even fewer.
I do wonder if to some extent that's cultural as in the SE guards in the past didn't do tickets, whereas in the North they always did and tend to be very particular about squashing it in even between close together stations. You have something similar with London buses, where drivers tend to just grunt at any attempt at a question and ignore any matter of revenue, whereas elsewhere in the country they do speak to you even if it's to say they don't know, and they very much do pay attention to tickets.
North of Northampton is also far from perfect in my experience, and I'm also not sure that conductors always only work in one direction from Northampton. However, a fellow passenger who recently engaged me in conversation said that, to cross the Northampton boundary, they had to ask the Union; unlikely I know, but where did that idea come from?
I travel fairly frequently, and LNR (and, for that matter, VWC and XC) conductors are very rarely to be seen between New Street and Coventry when it's busy. Fairly rare between Coventry and Rugby. Of course, there's the Centro very cheap day ticket (?£7.50) that's valid on trains, so possibly there isn't much fare evasion, particularly now there's barriers. The earlier morning down services (06.00 - 07.00) are exceptions, and do seem to be checked punctiliously, though the cynic in me wonders whether the likelihood (very high on one particular train) of TOC management staff travelling on these trains is a factor.
It is hard to see, on a train from Northampton to Euston that only makes 1 or 2 calls, why conductors are 'too busy' to do fare checks; and, when they
do do checks, why the excuses of passengers in the first holding standard class tickets are so readily accepted (there's quite a few clues - the word 'First' appearing all around, for instance) - some don't even ask them to move, particularly if the conductor is male and the passenger is young and not!
But, as you say, there are those who do try very hard indeed.
I'm not sure whether it still applies, but from conversations a few years ago, conductors then did receive commission and were expected to achieve a minimum level. One I spoke to had just sold a WST to Reading, and, as it was Monday morning, he calculated that he could take it easy for the rest of the week.
All of this is in complete contrast to the Midland Main Line, where there is a similar number of calls but tickets are checked very frequently and first 'policed' very actively.
