With mine, As it lost some, some baby ones were coming in.
The most common reasons for an Urchin to lose spines are:
1 - Osmotic Shock - typically from a rapid decrease in salinity. This can be something as simple as adding a weeks worth of top-off all at once.
2 - Starvation - they are fairly easy to feed. Try an algae sheet.
3 - Low Salinity. Depending on the species they do not tolerate low salinity well
4 -High nitrates (above 20)