An Approach Controller working arrivals to monitored finals reported that two aircraft failed to change frequency when instructed; resulting in a system error.

Narrative

I was working final for Runway XXR. I instructed Aircraft X to 'contact Tower 12C.DE'. I think I even used the word 'now.' Final Monitor called me about twelve miles later and asked me to switch him again. He was side by side with [a Runway] XXC arrival. This happened twice during this session. The first time was about fifteen minutes earlier. The last time this happened to me and the pilot failed to perform as instructed; I was given a Category A error. This was a stupid result of course because both aircraft are doing the same thing whether on the right frequency or not. This was totally a pilot deviation error. The pilot was instructed to contact the Tower in each case. With Aircraft X I told him he was supposed to be on Tower and he said 'I thought you wanted me to do it at [the marker].' Other facilities may be using phraseology such as 'contact Tower at' or 'monitor Tower; report' and this may be contributing to the problem. It might make sense to standardize this operation across the NAS but; in our case; I don't know how we could be any clearer than 'contact Tower' and in at least one case I said 'contact Tower now' and he still failed to comply.