Problem with Google xml sitemap plugin

Hello,
am Having problems with creating xml sitemap for one of my site. this is the message am getting from google xml sitemap plugin “There was a problem writing your sitemap file. Make sure the file exists and is writable. Learn more
There was a problem writing your zipped sitemap file. Make sure the file exists and is writable. Learn more
The building process took about 0.05 seconds to complete and used 45.5 MB of memory.”
please how do I fix this, I have changed the xml files permission to 777, but the message is still there.