I'm so sorry @Marchintospring and I know it seems shit. But my 2 (two years between them,) gave me fuck-all cards between 17-21-ish. No mothers day cards or birthday cards. Nothing for me and DH's anniversary - OR any Christmas cards either. As little school age kids, up to about 13/14, they used to make them, and it was lovely, but by late teens to about 20/21, they CBA. As a pp said, they are very self centred, and care much more for their mates and attracting the opposite sex (or the same sex even.) AND they have many things going on in their lives, including a new and exciting social life, loads of new friends, and uni work/assignments etc, and sometimes some travelling/ backpacking.
I lost my parents before I was 35, and both sets of my grandparents by 15, so when my 2 kids couldn't be bothered with Mothers Day, I was very hurt and upset. No card off them 2 - and no phone call or visit (when they were at uni.) I had no mother myself either. So Mothers day was very sad for me for 4 or 5 years. (Same with Fathers day as DH lost his parents by his early 30s... so he was a little hurt too.)
However, by their early to mid 20s they started to get thoughtful again, and started to get nice cards and a little gift, and never overlook/forget it now (or fathers day, Christmas, our anniversary, or birthdays.) It's just a stage in their lives when they're really self absorbed and selfish, and their parents barely register in the top 10 priorities in life.