Tuplet not working

I'm new in using Musescore. I'm always using the tuplet (particularly the septuplet and others). Here's what I did: I clicked on a note, pressed Ctrl+7 (Windows). But I cannot edit the tuplet, nor add notes to it. I was in the middle of writing a long piece, when this happened. I tried to close and re-open Musescore several times, yet when I opened the same file, it happens again. But when I started a new file, the tuplet is normal. What shall I do?

Thanks...