What version is being used? Test the latest beta. For licensed users upgrades are free for up to six months from purchase date. After that, VP will continue to fully function but a fee will be required in order to register the newest. Retain your old install file and registration info.
This can depend upon how the file was created. Share it, and someone will check it out. Upload the file/s to a free server, such as Google Drive or MS OneDrive, get a shared (public) link, copy the link and paste it here or to me in a Private Message via the mail envelope in the top-right corner of this forum. When using Google Drive, if necessary change "restricted" to "anyone with link can view."
To convert a single-channel file (likely the left channel) to center-channel mono, use Wavepad. At the bottom of the window click the caret next to this ... 
Then choose MONO.
Save the file and substitute it for the original in Videopad.