I'm not by any means an expert in dying, having only done it a few times in various classes. However my understanding is that Synthrapol is supposed to pick up the loose dye particles and keep them from adhering to other fabrics. It shouldn't be able to remove dye that has set into the fabric, so I suspect that the problem was that the dye was not set.
One of the cautions on the Dharma site is this: Fabrics treated with permanent press, conditioners, sizing or water proofing, etc are not recommended. I wouldn't be surprised if the shirt collars were treated with one or all of these to give them more body. Unfortunately that would also interfere with the dye.