I have a double sink in my kitchen. The pipe lacks any physical clogs, but continually gets an air blockage. There is a cleanout (just an unscrewable PVC cap), that when opened, everything flows wonderfully.

I've put an air admittance valve where the cleanout was with no changes. It seems that instead of a vacuum, an air bubble is building up and causing the blockage.

I have no issues in any other piece of plumbing in the house. The issue just occurs between these two sinks. No changes have been made to the sink itself since I purchased the house.

I've attempted to find an air release valve, to do the opposite of the AAV, but I've had no luck. But opening the cap every few days immediately remedies the problem.

Any experience with such an odd, isolated problem?

Here is a basic drawing of the plumbing.

 

Your AAV needs to be at the height of the drains, not low like pictured. With it low, it will lock up and prevent both air and water movement, negating its purpose.