Can you send a sample of one of your files with which you are having an issue?
What codec / filetype / resoution are you using?
Most of the time - it is due to content encoding - unless you are running a particular visual effect or massively scaling the image...
can you send a screenshot / picture of what you are seeing.
If you need to organise some training to help get your heads round it - do get back to me.
Regards
Simon