Try to force the distribution agent to see if it is still working or if shows any error.
You can do this by running the Job of the distribution agent in the publisher (for push subscriptions) or the subscriber (in the pull subscriptions).
Also can run the agent by executing distrib.exe. (usually in c:\program files\microsoft sql server"\80\com). Check in BOL for parameters.
If you find any errors after running the agent, post them here so we can help you.