Presentation in PHOTOS not working

Are you asking about a Photos Slideshow?

Are the videos in your Photos Library compatible with Catalina? Catalina is no longer supporting all video formats. You may need to convert the videos that are no longer playing to a different format. Here is a list of media formats, written for iMovie. It will give you an idea which video formats you can use on Catalina : https://support.apple.com/en-us/HT209029

Most of the other themes need a long time for the transitions and cannot play short video clips.

Have you tried to set the duration of the slideshow to a longer time? The default value is set to a very short time.

I am always using the Classic theme , because it is the only one, where the photos and videos will not be cropped in a silly way and we can control the timing of the individual items.

Only the Classic theme seems to be respecting the duration setting. The other themes are currently hopeless when playing videos.