Investigate and Identify Issue with Magento Website

We have a Magento installation operating with several websites. The sites work fine, but when a new product is added it doesn't show on the frontend, nor does it show in the admin... nothing seems to happen! However if you go to Manage Categories and try to find the product in Category Products it is there. So the product is adding to the database, but it's not showing in the admin product list or the frontend of the website. We need someone to have a look at the code and the error logs to try and identify what the problem is. If you can find the cause of the problem and you can fix it we will make a decision about whether to try and fix it ourselves or allow you to do it for an additional charge. At this stage we just need someone to investigate the code and identify the cause of this problem.