i am synched, yes. why would my wallet show an amount but not send it?
Have you ever exported any private keys from your Bitcoin Core wallet and imported them anywhere else?
Have you ever imported any private keys into your Bitcoin Core wallet?
If the answer to either of these questions is "yes", then it is possible that transactions that were not created with Bitcoin Core (and therefore Bitcoin Core was unaware of the change in balance) effected the "real" balance available to your Bitcoin Core wallet.
Also, if you recovered a backup of Bitcoin Core, and tried to send a transaction before the wallet was fully synchronized, it is possible that your wallet was not yet aware of some transactions that were sent after the backup was originally created.
If you created a backup of a Bitcoin Core wallet, and then tried to use that backup in more than one Bitcoin Core wallet, then each of the wallets will not necessarily know what the other wallets are doing with the balances.
If you haven't done ANY of the things listed above, then your wallet seems to have become corrupted somehow. You may need to recover a good backup and rescan the blockchain.