I've run into a similar problem and found that the washing in dishwashing soap would clean the surface, but wouldn't do a complete job of degreasing it. I also suspect that some soap brands actually leave deposits.
An easy way to determine this is to pick up a small bottle of Simple Green. First, wash the part in dish-soap as you currently do and after rinsing make note of how the water sheets off the part. If the rinse water beads up, even slightly, then you have a surface contaminant present.
Take the Simple Green and spray it completely over the surface and scrub it with a toothbrush and rinse. The water will completely sheet off the part, and if it doesn't repeat the simple green treatment. If you notice an improvement in the sheeting action between the dishsoap and the simple green cleaning, then you can reasonably guess that the dishsoap is the culprit.
I'm not sure if Simple Green itself will cause any issues with the anodizing process, so I follow it up by using Caswell's degreaser.
My process is to wash in dishsoap, rinse, wash in Simple Green, rinse and then use Caswell's degreaser to make 100% sure it is thoroughly cleaned and degreased.
Another potential area to investigate is that you may be getting some shadow effects where the electrical field is weak in one area due to placement of the anodes/cathodes, or any other item that may be interfering with the electrical field.
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