Appending to file, getting error file is being used by another process;

I am using a function posted on stackoverflow to check if a file is locked and append contents from another process.

http://stackoverflow.com/questions/9394629/how-to-check-if-file-is-being-used-by-another-process-powershell

But everytime i check if file is locked and proceed to update it says the file is locked? 

Thanks