Make Sure You Have An Available Balance
A wallet needs to have at least 1 Vulc available. It will also have to be well confirmed. A balance with 0 confirmations for example will not be able to start staking.
Make Sure Your Wallet Is Fully Synced
Check out this thread for help with syncing issues :
Make Sure You Unlock Your Wallet Correctly
A common problem is a person may unlock their wallet but not wait until their wallet is fulling synced. Make sure you unlock your wallet after its done synchronizing with the network. This includes both catching up to the latest block and synchronizing with masternodes.