Excel Lock files

Here is the story.  We have many server but on two servers that we use as NAS devices we are experiencing a lot of excel files that are show has opened and locked.  Sometimes it keeps a ton of .tmp files on the NAS but doesn't delete them when the file is closed and sometimes there is nothing open but the file still says access denied when trying to delete it.  It also says that it is in use by another user when you try to save it to the NAS.  Most of the time the person that says is using it is usually the person that is trying to save it the file.  The correct permission are on all the folders and have been there for 3 heres.  It has been just recently that the lock files have started.  
My question is could it be somewhere in the Switches because I show no evidence in the network where a file is held up or locked somewher.  Just a reminder the folders have the permissions and the files show no *.tmp file but it still shows lock.