A interesting issue has been found where people are reporting that the amount of coin sitting in the the node private address for challenges is reporting 0.
This has got to do with the way the zend daemon performs the challenge.
If you put 1 ZNT into the private address for the securenode, when the secure node tracker software executes the challenge, the zend daemon takes the "note" (complete 1 ZNT) for the challenge, before returning the "change" back into the wallet. And should your node be performing the challenge while a block is found, in the next block your secure node tracker software may report you have 0 balance for challenges.
To alleviate this issue, and after testing it myself, it is recommended that you put 1 ZNT into the challenge private address in 5x 0.2 ZNT transactions. By doing this, the securenode tracker will not spend all of the ZNT in the challenge address and there will always be some ZNT in the challenge address should the challenge cross into another block.
I hope this helps, and should you have any questions post them below or hit up the #securenode channel on slack.