As outlined in https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=13498#c16 several slips do not set at transport type and thus fall back to use the _email_ template even though they are meant to be pritned and the print template may even exist. Given Bug 13498 is quite long standing and there might be no easy solution as outlined in https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=13498#c17 it might be worth a notice in the manual for the slips affected. (Reason: it is not clear to the uninitiated what's going on if she is working on a template that seems to never end up in the system. "But it's there. Why?!")
One affected area is e.g. https://koha-community.org/manual/latest/en/html/circulation.html#receiving-holds