Tested a WAV file where volume had been reduced by 50%, and an Mp3 amplified 110%. Could not duplicate your result on a low-end PC with the latest free version. Worked fine.
Suggest uninstalling VP via Control Panel|ADD/REMOVE PROGRAMS, then installing from the above link. If prompted when the program launches, click on LITE VERSION.
Better still, purchase Videopad and register it, so you can take advantage of all features.