What version of VP is being used? Test the latest. 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.
If the problem persists, share your project. Someone will check it out and likely come up with a fix. The process isn't difficult and takes but a few minutes.
Here's how... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/
If it contains sensitive material link it in a personal message (PM). Click the envelope above. It won't be passed on or retained.
Use FILE|SAVE PORTABLE PROJECT AS, not simply SAVE PROJECT, and don't forget to share it.