Which services are supposed to have an at seat service and which do is not necessarily the same thing.
In my experience the trains on the Liverpool to Nottingham section which are advertised to have an at seat service do and the staff make every effort to come through, even on very busy trains. It is extremely rare for me to travel beyond Nottingham so I cannot comment on that section.
On the Sheffield to London trains, all weekday services should have an at seat service in First and I have not encountered one which didn't. In standard they are supposed to come through with a trolley but whether they actually do is variable.
EMT don't seem to know what they want to do on the Saturday London trains. All the buffets were closed and supposedly replaced by a trolley service which, in practice, was just as likely to not appear as appear. They then reversed the policy and reopened the buffets again but stopped the at seat service in First as well as standard (except for a very small number of breakfast trains). I have not travelled on a Saturday since the recent timetable change but the EMT website seems to suggest that the trolley service is back - whether that means the buffet is now closed again ??