Last couple of cooks been struggling to keep it at 120c

Messed up a nice pork belly because I could not get below 150c.

I wonder, found a deal online for two bags of marabu charcoal(picture above). They are however very small pieces and im thinking it might be that?

Or should that not matter? Before i mostly used KJ Big Block, i don't think i changed my routine for low and slow.

I fill up the basket, light it in the front with my torch, close the Joe, all vents open, then when im around 100c i adjust the bottom vent to 2cm and the top on the first bar.

I overshoot it and i cant get it down.

Any tips or tricks are welcome, i will give it one more go and otherwise I'll just keep the coals for hot and fast stuff.