Note only plays when saff above is empty

I'm writing this song where I have an eighth note that connects to a dotted half note, with the eighth note coming first. The connecting line is over a barline or whatever you call those lines in between every measure. It's on the second staff (both are treble clef). It plays like it should, but then when I add in anything in the first staff above it it doesn't play right. Whenever there's a note in the first staff, the note in the second staff is played like an eighth note when it's supposed to be 2 3/4 beats. I had to make a second staff because the first one had the same problem when I made two parts, so I thought it was just too much information. If I copy and paste the eighth and dotted half note somewhere else, they work okay. But then if I copy the notes in the top staff over above where I pasted the eighth and dotted half note, the two connected notes still only play as an eighth note. I made a new score, uploaded it and downloaded it, restarted MuseScore, and restarted my computer, and nothing works. It still has the same problem. The handbook doesn't cover this. This is an error that's not supposed to happen.