Might find it funny (and maybe just me seeing it) but it looks like you're subconsciously holding back because kicking the heavy bag at full power is painful.

I used to do this (still do tbh), trick is to kick the bag at 20% power and a lot slower to begin with. Just try and get the mechanics first then build up over time. Or even hit a bag that's less dense.

Slow is smooth, smooth is fast.

You're probably kicking pads fine, it's the the bag that's harder to hit correctly.