Since most other mechanical things have been tried, let me try another tact. Is your longbow cut about 1/8-inch proud of center? (The other bows you mentioned you shoot are cut past center.) If so, and if you aim by looking at the target only and don't use split vision, then your brain is placing your bow hand where it always does. However, with the longbow, the arrow will be pointing to the left of the target rather than at it. Of course, the arrow will then hit left. If that is the case, just have to shoot until your brain adjusts to the change. However it will be difficult to do if you mostly shoot your other bows. Good luck.