There are probably a few things to think about. I'm going to guess that your back yard faces south and possibly has less tree shade from the east and west. If your front yard faces north and slopes down, then that yard is at a very low angle to the sun much of the year.

Another thing to consider is the seed mix. The standard approach is to water briefly, 3x per day, until the seed sprouts. The problem is that fescue sprouts much sooner than KBG. Many times the home owner will stop the 3x watering before the KBG sprouts. If you want to have a thick, lush lawn, KBG is what you want.

Another possible issue is covering the seed with mulch when you seeded. That often works but what usually works is simply rolling the seed down with a water fillable roller. The idea is to keep the seed moist longer. Both methods do that, but when you cover it with mulch, the sprouted seedlings have to grow up above the mulch before they can photosynthesize. If you just roll it down it can photosynthesize as soon as the leaf forms.

If you mom has thick turf it is almost assuredly KBG. KBG is a sod forming grass whereas fescue is a bunch grass. In addition the angle to the sun and shade might be major factors. Each fescue seed makes one plant that essentially does not spread. It widens but not very fast. KBG grows out and across the soil to form a dense turf. Both have their uses, though. If you have a low sunlight situation in the front, you should probably have fescue in front because it can thrive on less sun. You will just have to reseed more often (annually??) to make it, and keep it, dense.