Missing Injector inconsistently

I'm using Cayenne 4.1 in the integration I took over to make it just 
work with Apache Tapestry. I'm getting:

Can't attach to Cayenne runtime. Null injector returned from 
CayenneRuntime.getThreadInjector()

The really confusing part is that it works just fine in Jetty, and it 
sometimes works in Tomcat. With one project it usually doesn't work in 
Tomcat, and in another it appears to work just fine.

So I'm really confused as to what is going on. Watching through an 
attached debugger, I don't see why it is or isn't using a runtime that 
already has the injector provided. The integration was created by more 
talented people than me for Cayenne 3.0, and I see this injector stuff 
is starting from 3.1. So I'm probably missing something.

I saw a previous post to the list talking about adding a filter, but 
that doesn't seem to match what I need here. I probably do need to 
change the service that is creating the contexts to always setup an 
injector??

Any pointers are appreciated.

Richard