What can I do about my lawn growing unevenly/patchy?
This depends on the type of grass and other conditions where you live.  I live in Houston and have St Augustine grass.  The following are common for my conditions.


Take-All Patch (Root Rot) : Very bad! Not much known on how its caused.  The roots just rot away and not a lot you can do about it. If you can reach down and pull a little and the rots come up, this is probably what you have
Brownpatch : A fungal disease that is really not an issue this time of year. It usually is caused by too wet conditions and cold nights.  
Insects (Cinch Bugs) : Really bad in the summer for lawns that are poorly watered.
Watering : maybe you have a sprinkler zone not working? But if you have several smaller dead spots then its probably not watering issues.


Maybe post some pictures.  Also you can take the pics to an independent nursery in your area and they might be able to help.  Big Box stores aren't much help, but a small nursery with knowledgeable people can help identify common problems in your area.

Have you applied any particular fertilizer lately?  Any other products ? 

Remember, I only know St Augustine, so if you have something else the above might not apply.