I have a 2014 vintage Sansa clip sport 8GB. It has developed a fault in that if I try to change to the next track whilst in shuffle mode it turns the volume down. I can revert to a previous track but not go forward. I have tried a reset ie holding down the power button for 20 secs and tried downloading the latest firmware but it appears to be up to date.The unit is only one year old and so this is very disappointing. My old Sansa never let me down.
it sounds like the buttons are broken. contact sandisk support to check the warranty status of the player.
Did you manually apply the latest firmware (even though you already may have it)? Sometimes, glitches can develop over time.
Also, if you are still within 1 year of purchase, you can contact SanDisk for a warranty replacement. https://www.sandisk.com/about/contact