directoryinfo.getfiles return 0 elements

In my process, i use the following code

DirectoryInfo dir = new DirectoryInfo(path); // path is a network path

        FileInfo[] files = dir.GetFiles(".xls");

actually there are 2 .xls files in that path, i can see when i paste the path in computer explore. but in the program files length is 0;

I tested in my local computer it can get the 2 files, and files length is 2.

but when i deployed in server, i set a schedule task use the same domain account as on my local. from the log i write, 

the files length is always 0. it runned many times,still 0. 

the trick is, when i restart the server, the can get those 2 files.

what will be the possible reason?