Black Screen after export at the 4:3 16:9 sequences

Hi guys,

im quiet new to Shotcut so maybe i gotta say sorry for a dumb question first…

My problem: i made some videos with my GoPro, some of them are 1920x1080 and some 1920x1440. So i have put some videos together and exported them. I expected that there will be some black bars at the 16:9 parts, but these parts stay black now (completly black screen)…
What have i done wrong?
Thanks for the answer!