They aren't running low on conscripts. They ran low on people they could conscript using the previous set of conscription laws that were extremely "leaky". Then they changed the law, and now they're not really low on manpower anymore.

Ukraine is running low on conscripts because they're not drafting 18-20 year olds.

their national birthrates, just like in the rest of Europe, are low, and losing 20-40% of the 18-25 year old cohort means population collapse.

Russia is in a similar place, and is generally only drafting from ethnic minorities, far eastern locales, and prisoners, plus a hearty dose of mercenaries. that said, they have 3x the population and can just pull way more people.

in both countries the average of a trooper is like 38-45.