I've been building medieval style crossbows a long time, so I have a lot of opinions on the subject of roller locks. Take them for what they're worth:
1: your roller nut should fit closely in its socket and turn Very Easily. Heavy rollers have more inertia: they accelerate more slowly and particularly if they're tight in the socket, that can contribute to misfires... the string climbs right out of the lugs. Lower lugs, mounted ahead of center may have slightly more tendency to misfire. They may also release a bit faster, which is good. Most medieval bows I have examined, have the back edges of the lugs approximately in the center of the roller. A few, like the Maximlian Bows in Vienna, appear to have rather longer lugs, that pass behind the top-center.
2: Payne Gallewey shows his roller nut with @ 3/5 of its diameter below the top of the stock (in a steel socket) in my experience that's too shallow. I recommend sinking it by 2/3. As one approaches 3/4, you can no longer remove the roller from the top of the socket... you'll have to take it out the side, which means you have to drill your roller-socket from the side, rather than using a mortised lock that goes into the top. PG also places his lugs at top center and his iron sear at bottom center. Given a relatively shallow nut, particularly with a slightly undersized roller, the nut may try to rise out of the socket under load, which increases wear on the edges of the socket. The socket can be reinforced with bone or metal, which should deal with the problem, but I have discovered moving the sear-plug back about 1/4 inch reduces the load on the edges of the socket, transferring it to the trigger. Of course moving the sear can make the trigger harder to pull, but that can be remedied by careful adjustment of the sear-face and the end of the trigger, so that the trigger end (distal end) drops straight down out of the sear, rather than forcing the nut to roll back on release. Many bows made in late 15th century used a two-axle lock that carried the sear plug on the Back of the roller rather than the bottom. They work fine that way.
3. Bows that tend to misfire... particularly ones in which the string climbs out of the lugs on release, going over the string (may mean you have too little down-pressure on the string or a string that's too narrow, or groove is too deep... the possibilities are numerous) Sometimes misfire can be cured by cutting a hollow in the backs of the lugs... make them concave. Some medieval bows are done that way, but certainly lots of stronger bows with very fat bowstrings just had flat surfaces on the backs of lugs. Again, a heavy roller tends to exacerbate misfire problems. My first crossbow had a heavy, wide brass nut. I had endless problems with it.. eventually cured most of them, but the thing still misfires occasionally if I use a bolt clip (another set of problems there... if your bow misfires with the bolt-clip but shoots fine without, that means the string is rising out of the lugs, but carrying the bolt with it. When there's no clip, it may shoot pretty well. With the clip in place, the bolt is stripped off the string, which goes over the bolt, throwing it into the ground. Endless entertainment. Everybody has to start somewhere and inevitably everybody runs into these problems. Persevere! Geezer.