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