Noob in need of help spot welding to the aluminum body of a LiFePO4 26650 cell

Hello, this is my first post on this website, so let me know if I'm doing something wrong. I am currently redoing an old pack I made when I was 15, (held together with hot glue and duck tape) to have a 3d printed case and spot welded connections instead of wires soldered directly to the cells. I finished the case, but am now trying to weld to some cells that refuse to stick. I think I understand the problem, but sadly not the solution The interesting thing is that the negative terminal welds fine, but the positive terminal remains untouched after the nickel strip melts. I believe that the positive terminals do not have a 'tab' (still unsure about the proper vocabulary) so I can't weld the nickel strips directly to the aluminum body. Does anyone know of a way around this? Or am I making a dumb mistake? Thanks in advance,
BeanTime

Video of welding each side:
https://www.youtube.com/watch?v=ZUQYa5fN-n0

There are a number of possible problems.

The can material may not be weldable with the process or material being used, but lots of stuff has been welded, so that's unlikely. Based on my own years-old experience with even-older LiFePO4 cells (18650) they were unplated steel cans, and that may affect weldability (it certainly prevented soldering in the case of the repairs I needed to make ).

It could be the energy is simply insufficient, or in the wrong pulse shape or timing, to get the heat buildup needed. This could happen on only the one end because the other is a thin plate not directly connected at the weld point to anything else that can take away the heat, but the problem end has the internal plates and electrolyte directly connected to it to soak up heat.

There could be some other problem; you may have to look at the various welding threads discussing welding of various materials to various cells. (sorry I don't have any links, but the Kweld threads have some info on this).