Sandpaper stops cutting halfway through flattening plane sole

[Vanya]

Working on flattening this old stanley no. 5 sole with 80 grit on granite. So after much ado actually, the blade assembled and retracted like it should be
But the problem is after about 15 plus mins, the sandpaper just stopped cutting the sole even though it's not flat yet. I can see sharpie marks so it shows the middle is getting worked but the toe and heel barely touched and the paper still cuts other metals fine, just not the plane sole anymore, different pressure and motion patterns don't help either

What's the solution here?

 

[Handym4n]

hi Vanya! looks like your sandpaper is dull or just loaded up with fine iron dust and needs to be replaced

 

[bridger]

You don't say what kind of paper you're using. Some will stay sharp long enough to fail by clogging up: those will benefit from frequent cleaning. Other kinds dull more quickly and need to be replaced more often. Either way no sandpaper lasts forever.

 

[Alan]

Probably got a convex sole

 

[Niloc]

Use emery cloth and finish with wet and dry to get a fine finish.