Why is my cleavage uneven? 6 months post-op.

Dear becca585,it is hard to tell without an examination. I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery

I am sorry to hear about your concerns after breast augmentation surgery. Your plastic surgeon will be best resource when it comes to accurate assessment, advice, and/or meaningful information.  
Generally speaking, there are many variables when it comes to the final appearance of breasts after breast augmentation surgery.  The main variables, in my opinion are your anatomic starting point, exactly how/where the breast implant pockets were dissected and the size/profile of breast implants utilized.  Again, for this reason, your plastic surgeon will be your best resource.
 
Generally speaking, breast implants will "drop" into the pocket that was created during surgery. The rate of breast implant "settling" may depend on factors such as size of breast implant pocket dissected, tightness of the overlying skin/muscle layers, and size of breast implant utilized. Normally, it takes about 3-6 months (for some patients longer) to see the final result.  As the breast implants "settle", the lower breast poles will round out, and the nipple/areola complexes will seem to sit higher on each breast mound.
 Careful communication ( preferably in front of a full-length mirror) of what you mean by "uneven cleavage" will need to be carried out to precisely identify your concerns and (possibly) plan an operation to achieve an outcome that you are happier with.
 
Best wishes for an outcome that you will be very pleased with.