Word Automation Service fails

I am new to SharePoint and facing this problem in SP2013 I have used Word Automation Service which helps me to convert the Word file (which is actually a Template to which I am adding my data from various lists) to PDF using ConversionJob & ConversionSettings. this process works well for certain time limit without any error and i get expected output, the issue is this Conversion from Word to PDF fails suddenly and Word files created are not converted, there is no issue with generating word file. i have tried various ways but i am not able to trace the problem can anyone help me out or suggest ways