Hello Carol Hearn -
Thanks for reaching out to us about this issue. We are aware of some intermittent difficulties when searching by ISBN in the Find Product search which our engineers are looking into. Please be aware that this search does pull data directly from eBay, so at present it is unclear where the difficulty lies.
If you have specific examples you would like us to have a look at, please feel free to submit a support case from under the Help tab of the site letting us know the titles of some of the affected items so we can include that information in our efforts to determine the cause.
I will update this thread when more information regarding this difficulty become available.