This is most likely the case. If your wallet is not synchronized, that means that it does not yet have a record of all transactions. Since synchronization happens in the order that blocks (and thus transactions) occurs, your later transactions will not show up until the synchronization reaches that point, which could take a while...
If you want to confirm that your coins went through without having to synchronize your wallet, you have at least two options:
Web-based Wallets such as BlockChain.info are connected to a backend service that is constantly synchronized. With blockchain.info, you can search for the public address to which your coins were sent without having to go through any signup process. This is probably your easiest option.
Electrum (http://electrum.org/) is a wallet that connects to servers that are already synchronized and thus will give you immediate results as well.
Also, there is a vague possibility, that this issues is caused by something other than the synchronization, so, if the options above don't work, please update the question with more details.