Are you able to put a couple of scrren shots up of your mail settings please. The mapping and the general settings as well.
If you are getting some emails but not others than it is more likely a problem with your process. Make sure the reminder is set to be active and that the role is specified correctly
As always the knowledge that flows through the space between your ears could be bottled and sold at a high price.
The assignment action instance data did not have "notify assignee" checked. It did have notify originator checked, although.
Careful, or we'll have to buy him a bigger cork!