FM is limiting how many photos I can add!

I saw a post where someone made a slideshow with 478 photos and it was about 15 minutes long he said in his post. Well I need something similar and it’s stopping me from adding anymore photos and I’m at about 278 photos on the timeline. I need 350 in my slideshow. Why am I being limited? I have enough music laid on the timeline to go up to 400 photos if needed. When I drag a selection of photos down to the timeline it just vanishes. What if anything could I be doing wrong?