This can happen if either the sender or receiver is forked, not synced, or otherwise not aware of the transaction, but either way there is no need to worry.  If the sender had a problem with their wallet, they will still have that money when the problem is resolved.  If the recipient has a problem with their wallet, they have the money and they just can't see it until the problem is resolved.  


To determine where the problem is, check if your transaction appears on the block explorer by looking up the transaction ID or the destination address.  If the transaction appears, the problem is with the recipient wallet.  If it doesn't, the sender has a problem they need to fix. 


The wallet with the issue can find out if they are forked or unsynced and fix it by following this article: https://pivx.freshdesk.com/support/solutions/articles/30000004667-my-staking-or-masternode-reward-is-much-higher-than-expected-and-or-my-recent-transactions-aren-t-sh