the "three pronged mount" you described is known as a "spider". why i don't know, but it comes on alot of drum thrones of various qualities. it doesn't mean its cheap, it only means its a item commonly used.

anyway, the only thing i can think of is the throne base/post is not the right size that fits that spider. the usual problem i come across is that the throne post is smaller or bigger than the "spider", and vise versa.

or you have the post locked on the tripod or the t-handle screw under the "spider" is screwed down or causing the seat to freeze up.

The black plastic screw handle is backed off as far as it can go.

I watched the guy I bought it from put it on when I got it from him.

He didn't do anything special.

Hmmmm

Thanks for the reply drummerfish!

I am going to try some WD40 on it.

try looking into the spider hole to see if the cast metal is worn, cracked or broken thru to the wood. or if anything needs to be cleaned out of it.

The seat spins freely but when I try to lift it off, it sounds like its hitting something but I have the screw as far out as it can go.

I can't see inside the hole because its still on the spindle.