Search:

Re lap your SG20, some do not stay flat and smooth, bevel or round the edges, make sure all the previous stria is removed prior to going to the 20K. there should be no deep stria on the bevel.
...

What is on the paddles and are you using them?

Balsa has a reputation for harsh edges, if you look at them under 60-100X you will see, micro chipping, especially if too much pressure is used.

So, it could be your beard…

Generally if there is an issue with the edge life, it is the razor, the stones, or the technique (the Operator), honing, stropping and shaving.

The two you can...