Slice of Sky strongbox - no mastery point ??
getting the loot, but no mastery point usually means you’ve already got it once.
if this is the first time you’ve ever touched the box (and you’ve double checked your achievement panel), then you should probably make a ticket with support:
https://help.guildwars2.com/hc/en-us/requests/new
70 ‘mains’ and waiting for more slots
| 61 Asura | 5 Charr | 2 Norn | 1 Human | 1 Sylvari |
| 61 Asura | 5 Charr | 2 Norn | 1 Human | 1 Sylvari |