Amy, when the VA provides you the ratings, they typically give you the justification of how they came to the percentage and what needs to be met to achieve the next higher rating.
A zero percent rating is not a denial, it just means they acknowledge the condition exists, however there was not enough evidence to warrant the next higher rating above zero percent.
Look at the rating for the Migrane/TBI and see if you have the documentation needed to meet the standards they are looking for. If you do not have the documentation and the conditions are still present, make sure you are beating treated by the VA for the conditions, you can then file an additional claim for the condition.
Continue to drive on and get them to rate the TBI/Migranes. Find yourself a good physiatrist in the VA system, they will get you the correct testing and physical, occupational and social therapy.
The difference between the 90% you are currently rated at and the next higher percentage of 100% is over $1100 per month.