I have done a few chicken rotisserie's and balancing is certainly the hardest part.
My usual routine is to get the bbq setup and ready to go but I balance the chicken before lighting. I also bank the coals to one side (on the same side as the chicken rotates upwards) so then most of the drippings can land in a driptray. Then I let it do 10 or so spins to make sure its balanced ok and not slipping round the spit bar.
I ended up purchasing a couple of counter weights aswell, they dont fully solve the trickyness but they make it more forgiving. Hopefully this may help in future.
Main thing is it turned out delicious though😁😜