For forty years every thousand years, or so, he was fully free, but then drawn back into the Seal on the Bore.

My theory/idea would be that Ishamael felt the attack coming and tried to get away using the True Power. We know that he, alone of all the Forsaken, understood the DO's true objectives and that he almost always had free access to the TP. The others would likely not have had free access, or even been willing to use the TP in that manner. They would have tried to get away or counter attack using the OP. That is, if they were willing to do so without the DO's express permission at the site of Shayol Ghul.

If LTT and his planners were devious, they wove into the Seal a kind of flypaper attribute, so that if the weave itself was attacked or if someone tried to channel within the weave they would be captured within it. That is assuming any of the Forsaken did try to counter attack.

Aginor and Balthamel were not high on the DO's list of favorites, they might have tried to escape using the OP and almost succeeded, much the way I assume Ishy tried to escape using the TP, but because they used the OP they were less successful.