Next Slide and Previous Slide Buttons

Gonna throw this thought out there...

You mentioned layering your previous and next buttons on top of one another.

I picture them being in the same space on the stage - overlapping physically.. That made me wonder if the next button was on top and so that is the action that was firing.  Is it possible they need some more separation on the stage?

For the Replay button, have a look at my most recent blog. I also am finishing up a Captivate tutorial which shows more in detail the effect of Replay and Reset on different items in Captivate:

http://blog.lilybiri.com/replay-or-reset-slide

To have a Next and Previous button on each slide there are 4  possible approaches:

1. Simple but efficient for pure linear courses: use the Playbar, and eventually take out all items you don't want. It has a Previous and Next button. This works both for responsive and non-responsive projects.
2. For a non-responsive project: put the Next button on the Main Master slide. Make sure that all used master slides inherit the Main master slide items. That means you need to use one of the 3 most recent button types: shape button, SVG as button, bitmap image as button. Keep the default setting and the button will automatically pause at the end of each slide, whatever its duration. Drawback; Next button will be visible from the start of each slide, no way to force viewing (which is IMO bad practice for adult learning). You can do the same for the Previous button, but may prefer to do so not on the Main master slide but on used master slides except the first slide. Another drawback; you are no able to hide/show the button on slides because it has no ID.
3. Pu the buttons on the first slide and time them for the rest of the project. Same restriction to the newer types. Advantage: button will have an ID and can be hidden/shown when wanted. 
4. If you really insist on the buttons appearing only at the end of the slide, you can follow your present workflow and have a button on each slide, or delay the appearance of the project timed button with an On Enter action. 

As Rod points out there is no reason whatsoever why a button with a Success action of 'Go to Previous Slide' should go to the nexts slide. Show the screenshots he asks about. 

It's very unusual to have a button that has been properly configured for a Go to Previous action to just ignore the action and Go to Next slide instead.

Can you please show screenshots of your setup with each button selected and showing the specific Actions selected in the Properties tab?