Your Bitcoin wallet contains a list of transactions that you have performed. If you restored an old wallet, your newer transactions can be missing.

What you will want to do is shutdown the Bitcoin client, restore your old wallet.dat file, and delete the block chain files (blk*.dat). When you restart Bitcoin, it will need to download the entire block chain again, but when it finishes, you will have all your transactions back and your balance will be correct.

Cheers,
Motoma