Takes time to get familiar with editing software, no matter how intuitive. To protect your work routinely use Save Project File AS, giving it a unique # or name, so as not to overwrite prior saves. This creates a VPJ control file (not playable) that usually resides in the same folder as the source files, unless you have saved it elsewhere. Double-clicking on a VPJ file loads the project. Clicking the chevron at the right of the SAVE PROJECT icon on the Home menu toolbar will reveal the Save AS choice.

If you make an editing mistake, immediately press <ctrl-Z> (EDIT | UNDO) to backtrack, step-by-step.
One way to approach your task is to first place a single slide on the timeline.
Then place the red cursor where you want the audio to begin. Assuming that you are using Videopad's narration feature, record your narration clip. It will be placed at the cursor point on the timeline. There will be a clip saved to the audio bin as well. If audio has been pre-recorded, drop the audio clip onto the timeline in the same manner. Audio clips can be dragged left or right, for exact placement.
Slides can be moved as well, or sized....
To lengthen or shorten a slide drag its handles - the [ or ] brackets that appear when the cursor is passed over the start or end of the slide clip.
In general, to move slides, click the slide and drag. Dragging left shortens the adjacent earlier slide. Dragging right will move all later slides in concert. If that group snaps back after letting go of the mouse, hold down <alt> while dragging. If it instead leaves a gap, it can be closed by right-clicking in the gap and clicking CLOSE GAP.
Once you are satisfied with the sync of this slide, Save Project AS, as outlined above. Then add the next slide to the timeline by either dragging it from the bin and mating it with the existing slide or right-clicking it in the bin and choosing PLACE ON SEQUENCE AT END. Follow the same procedure for each slide and Project Save AS. Then you can always restore the project by double-clicking its uniquely numbered or named VPJ.
This step-by-step method keeps things simple. Each slide/narration is in sync.
As you have seen, altering slide position or length after several are on the timeline can mess things up, as audio and video are not linked. However, they can be grouped. Select the slides and audio clips that need to be moved together, right-click on one of them and GROUP SELECTED CLIPS. Then, when a slide is moved, the audio clips will be positioned accordingly. But, watch out: if a slide is shortened, audio for that slide will be as well.
Good luck!