Translating some buttons doesn’t work

I have a problem in the course registration page where there is the login part for users who already have an account and the new user registration part. In the login part I can’t translate the Login button (maybe a previous bad translation appears which is “plugin”). I used Loco Traslate and I found the entry: I changed from “plugin” to “login” but nothing (I saved, synchronized, tried again, cleared the cache several times, changed browser …) … nothing! How can I do? thank you