That type of behavior usually means that the field calculation order (not tab order) is incorrect. How you set it depends on what version of Acrobat you're using.
I am using Pro X
Here's what you'd do in Acrobat 11, and I think it's very similar for 10:
First go into form editing mode: Tools > Forms > Edit
Then select: Tasks > Other Tasks > Set Field Calculation Order
Thanks so much this worked.