In case you use the NEO desktop wallet, I assume that means that either your wallet is not synchronised yet (if the two numbers in the bottom left corner are not equal) or one of your transactions failed. Failed transactions can be restored by selecting 'rebuild index' and then closing the application and restarting it, logging into your wallet and waiting quite a while before your wallet entry is rebuilt (best to just leave it on and go do something else in the meantime). After having selected 'rebuild index' the amount of NEO and GAS listed will always display '0' until the process is complete. I do not know how long this process takes, but it requires some patience....it might be an hour for all I know.