I have rod holders on my big boat and kayaks. I bought Attwood Fishing Rod Holder extensions but the rod holders did not want to seat properly in the extensions.They did not want to go down far enough to lock in position. It seems the Attwood extensions have a more tappered hole. Use sand paper or a grinder to reduce the size of the collar on the end of the rod holders  a little bit and your problem is solved. They still work fins on the original base mounts.

 

 

 

 

That's the main reason I use RAM and Yakattack accessories - they interchange and fit seamlessly.