I bought the adapter, put it on my Canon 50mm 1.4, no problem. I wanted to use my Tamron 24-70 but it was being cleaned at Tamron and only got it back this week. Took the adapter off and tried attaching it to the Tamron and it just wouldn't go.

I went into the large electronics shop in town. They took out a new Kipon adapter and Tamron lens, everything worked except my adapter. It's still under warranty/return and I'm sending it back. However, when I got home I tried re-attaching it to my 50mm 1.4 thinking maybe when taking it off I messed something up. But it went on the lens without a problem. Tried it on my 70-200, no problem. Tried it on my 100mm macro, problem...but I don't know why. I can't find any minor differences in the mounting parts. Why would the adapter be able to attach properly to the 50mm and 70-200 but not the 24-70 or 100mm?

Anyways, the new adapter should arrive Saturday. I will test it right away to make sure all is working well. Thought I would throw this question out there just in case someone has encountered this issue.

I really have no idea, but machining never produces perfectly identical parts - they vary within some tolerance range. Tools wear down, machining friction makes the metal expand due to temperature increase, tool pressure can deform the material, etc Could it be that some of your lenses are at (for example) the large end of the tolerance range and your adapter at the small end? Or slightly out of tolerance spec?

We make high precision mechanisms and our tolerances are in single digit microns and we've even seen problems caused by the specific oil in the bins that catch the parts coming off the automated machines - it's there to slow the parts down so they don't collide with each other forcefully and any variation in viscosity can lead to damaged parts. On some parts we spend more on cutters than on the part material itself because as the tool dulls the friction increases and we see tolerance issues so tools are changed after only a few parts. Every process has tolerances and it could explain what you're seeing - just a wild a-- guess on my part.