We do it on nights just because there really isn't much of a chance to do so when things are slammed in the ER. But days manages, they have a few more people, and they cover for one another's zone. Try asking if someone will swap covering for you, and you for them. Get everything done that might be urgent, let things that aren't wait, tell your swapping nurse all they have to do is listen for anything odd and go.
Our time clocks have a button for no lunch, if you didn't get off 30 minutes, you get "no lunch".
Keep a log of how often you are not covered so that you can leave, document that you asked the charge nurse to cover, that your immediate tasks were done and leaving should have been possible. If you rack up enough time, go to your boss and tell her you want back pay. Get your union rep involved if they say no. Send it to the labor board.
We for so long had so few staff on nights that lunches were really impossible, but now, we've been upstaffed enough that on nights where we aren't absolutely slammed, we should be able to take breaks. New staff or travelers seem to expect lunch breaks! (imagine that), and frankly, I don't have a good reason not to be able to cover them (except on really slammed nights). Sometimes you can change unit culture, by gentle pressure, and simply expecting to get your rights. I am slowly realizing that lunches are now possible! I'm just not used to it.