Please try to reindex data and clear cache in System / Index management - the issue may be related to incorrectly made reindex or cache clearing.
In case the issue remains, you should check if all required data is included in your .csv file. Magento edit product may fulfill it while product editing. Simply open the product for editing and find all required fields marked with * and check them all in your .csv file (all required data should have values).
Once all data is included, you may simply try to reimport it. In case you will have same issue with your tool, I can offer you to try Store Manager application:
http://www.magentocommerce.com/magento-connect/store-manager-for-magento.html
Free trial version is available there.
Let me know how it goes and if you need further assistance.
