Tig Weld Crack (no filler)

Anyone have any ideas as to why the part towards me fuses fine with no filler and doesn’t leave any cracks 8/10, whereas the part on the far side leaves a crack every single time with no filler, seems just about the same clearances/tolerances, really odd. Any input would be appreciated. Trying to weld both in the same program rather than setting up a chuck to center a part at a time and using filler.

Here’s the part with no crack

Here’s the part with a crack every time.

I suspect they may be just two dissimilar metals that don’t want to join. Even mig welded by hand and tig welded by hand leave a crack after grinded down slightly.

Thanks everyone for the help! We took all the pieces to our metallurgist, sure enough the rod had a high lead content, meant for being machined easy, not welded. The tube came back as a low carbon content steel, which is why that had a much easier time fusing to the base plate. The rods registered as 12L14, which are going to be replaced by low carbon steel rods. Thanks again