Changing note durations deleting other notes in bar.

Hi all,

New to using Musescore, and my main focus is the setting of liturgical music (setting lyrics to the various Orthodox troparia/kontakia to their respective tones).

I've tried uploading .pdfs of pre-existing scores, or just writing them from scratch as an unmetered liturgical score; and I keep having this problem where changing the duration of a note mid-bar will automatically delete other notes at the end of the bar, and it is driving me mad. How do I turn it off?

Many thanks for your help!