Index statistic zeroing

Hi.

After monitoring long queries I created index and the index statistic counter started to increase from 0 to 15, than stopped. I realized also that after reboot/restart all the index statistic counters zeroing and some of it stay zeroed despite of queries. Deleting the particular index with zero counter causing slowness, recreating obviously fix the slowness.
So the question is-why in some cases index usage statistic counter stop without obvious reason?