A.) Some rewards are exclusive to a spacific Level if you have not reached the level you can not redeem the reward.
B.) Some rewards are limited by the number of times you can claim a specific reward, that way giving all employees a chance to claim the reward.
If you have lost your rewards voucher, you can go to the rewards page and click on "My Rewards" it should bring up a list of all rewards you have claimed, click on the reward to view the voucher number.
Note if you lost your voucher and someone else has used it, it can not be refunded.