We have Snapmanager for exchange through which we take backups and retention is " Do not Delete and Backups" but still I see only last 10 days backups, with my knowledge I went to the volume and checked the snapshots.
There are 10 snapshots same as they are shown in the Snap Manager for Exchange which means if I increase more snapshot space I would see more backups in the snapmanager for exchange.
Snap reserve is not about how much you allow the snapshot to grow. it's about how much you allow the data to grow (and by that. reserving space for snapshots).
Two main things that might causes this:
A. snap autodelete (ther's a cmmnd as such follow by a vol name)
B. you have another job configured with less retention (like weekly one, or in another node in the DAG) that myabe deleted it. there was also a few SME bugs with the retention. but the first option i sueggested is very likelly to what you describe..
if that still not the solution. need to look on /etc/ems for what deletes it. if it's point to ZAPI look on the SME backup report to see if it's coming from there or not