Sanding floorboards

[Dora the explorer]

Hi I have hired a sanding machine a sand glider... I wish I had just hired the drum and edger the glider has 4 orbitol pads one looks like it not even making contact with the floor I'm using 40 grit and it slowly very slowy removing the varnish...in some places though the varnish is turning black and won't come off any ideas how to get it off? Thanks in advance.

 

[big-all]

Absolutely no idea buuuttt
if it is properly set up assuming its in a square or oblong formation you cannot have a light corner you can only have diagonally opposite too high or low ??
as it will rock on the lowest 2 pads as the pivot


but purely a guess as i have never used a 4 pad sander