Is it in 8 bit (or indexed color, or 256 color, or whatever)?

Is it actually a bitmap? Sometimes when I'm saving a tileset in PSP, I have Paint Shop Pro Image (or whatever else) selected, but I type in the .bmp, and it saves. But in that case it's like a .psp image renamed to .bmp.

Does the file still exist under that name? You didn't rename the file, but not update the tileset's info in JCS?

Did you used to be able to compile the tileset? If so, what have you changed since?