Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
@Bonzo Since the document is a sensitive one. I can't share it. But I have found out that the gs command running background of the not converted pdf using 100% CPU.
I am afraid I am not an expert or have had this problem myself.
I would say you need to track down the difference between the ones that pass and the ones that fail. Are the pdf files created with different software?
Are the original pdf files that fail a lot larger, try setting a density convert -density 300 in.pdf[0] out.png
You can use identify -verbose in.pdf[0] to see the details of the files.