I had this issue with RCBS, Lee and Hornady shell holders for the 6.5x55.
I had Euro brass, the rims are thicker and won’t go in.
I bought a shell holder for the 30-30, and the 6.5x55 cases slipped right in.
It does not have any markings on it except number 2 stamped in the groove where the shell holder is milled out for primer clearance. I think it is another Lee shell holder, but not sure.
This is the only shell holder that my RWS cases would fit into, even PMC that didn’t fit the RCBS would fit this one.
I tried several too.