.dll getting locked out

Im getting my codebehind dll getting locked out. On occasion and I cant find a microsoft solution to this. The dll in question is a local dll stored in the bin directory. So there should be no confilict between it and any other program. There is virus protection running on the machine but to my understanding it only checks any files when they are modified and the application has been working pretty much for weeks. Then it ceases to work due to the the dll getting locked. Does anyone have any ideas why this would happen.