What VP version? Try the latest. When you refer to "process" is this processing in preview on the sequence or when the project is exported?
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 no change, share a project that exhibits this behavior. Follow the usual steps as shown in an earlier reply.