Cute _
Video editing is resource intensive and this high-res project may be taxing your PC, which could explain your issue.
Lowest preview resolution was used here and it took about fifteen minutes before all files were cached - confirmed by a nearly solid green line displayed above the thumbnails.
Working with files this high in resolution is problematic. Videopad should do better if the files are first converted to 1920 X 1080 (or smaller) format. (Mp4 is a solid choice.) Quality won't suffer measurably. It's possible to use very low-res files for editing, to ease the VP and PC load, then substitute higher-res clips for the export using the right-click CHANGE FILE feature.
Music audio that was panned to the right channel has been centered. The audio of the first two scenes is mono, while music is stereo. Because of this, VP shifts the mono track to a single left channel when exported. This was corrected by converting those scenes' audio to stereo in another utility and reinserting them.
There was unneeded, redundant audio of the first clip on another track. And each clip edge, with a freeze transition, is normally a single, clean cut point that the Go to next clip edge buttons can locate. With an overlap transition there are usually three points. In this project, each edge instead had four cut points. ?? May be related to transition problems that were encountered.