Try removing the PDFs and load something in either LRF or EPub. This should tell you whether the problem is with the reader or with one of the PDF files.
Are you using the Sony Ebook library software to load you books? It may be a bit quirky, but I find that it gives more stable results.
It is also just possible that the reader might need a long mains recharge. New readers take a little while before the battery meter calibrates itself and gives an accurate reading. See this thread:
https://www.mobileread.com/forums/showthread.php?t=21337