Did your coins come as a very big number of small amounts? Do you try to pay to a big number of addresses?
If this is the case, then you should select less inputs (pay less) or consolidate your inputs first (send to yourself to have less number of smaller transactions).
- Open Coins tab.
- Select multiple inputs with small balance by holding "Shift" and Left click to highlight.
- Right click to the highlighted inputs and Select "Spend", you'll be transferred to the "Send" tab.
- From there, send to your own address and click "Max".... then, click "Preview", make sure the size is lesser than 100,000 bytes (100kb) in order to be valid.
Unfortunately you may have the equivalent of a lorry full of pennies, which is "too big" to fit into one block (in the blockchain).