Normally when I right-click my app engine project and choose \"Generate cloud endpoint client library\" using the Google plugin for Eclipse, it generates the client librarie
The issue tracker shows that this issue was resolved (https://code.google.com/p/googleappengine/issues/detail?id=9746).
Also @howettl posted above in the comments that this issue was fixed.
There were some questions in the issue tracking about remote servers being involved and the fix not requiring an update to the GPE. This appears to be related to the fact that there is a Google web service used by the GPE for generating some of the cloud endpoint code.
If this web service is not working (routing issue, server issue, etc), or you don't have a connection to the internet, you will not be able to generate (or regenerate) cloud endpoints for your project.
Also if your generation is slow, it might be due to your internet connection speed.