Dusty,
From what Ronan has said there is nothing wrong with the taper on his collet chuck.
It is well known that an unrestrained Morse taper tang, even when properly fitted into a socket, will work loose due to the repeated change in side load imposed on it by the milling operation. This is why milling machines have a drawbar to retain tapered attachments.
The important point is to only tighten the drawbar to the minimum amount to retain the tang.
Over tightening a drawbar achieves nothing other than to make it very difficult to remove the tang without a lot of brute force as Ronan experienced.
By the same mechanism a milling cutter will work its way out of an ordinary drill chuck no matter how hard it is done up. This is why we have ER, Clarkson or Osborn milling chucks.
Eric