A couple of things to check...
Firstly, is your total balance made up of a LARGE number of UTXO's? If you click "View -> Show Coins", and the select the "Coins" tab... do you see 10+ items listed? If so, the reason your full balance is not spendable is probably due to an increased transaction size from all the UTXO's being used pushing up the fees (as they are calculated on a "per kilobyte" in Electrum. If you have a large number of UTXOs showing on your Coins tab, you have likely received a LOT of "dust" sized transactions (cloud mining? faucet collecting?). More inputs = more bytes = more fees.
If you want to get closer to the total balance, you would need to disable dynamic fees and/or enable "edit fees manually" in the settings (Tools -> Preferences -> Fees). NOTE:
Doing this may result in setting fees that are too low for current network conditions which may result in longer confirmation times, BE CAREFUL!:
Also, check that whether you have "spend only confirmed coins" ticked (Tools -> Preferences -> Transactions):