Acceptance Transform Rules not firing

I currently have three Claim Provider Trusts defined:

1) Active Directory

2) Secondary ADFS Server / Active Directory

3) 3rd party Claims Provider

I have added the following Acceptance Transform custom Rule too each of these Claims Provider Trusts:

 => issue(Type = "http://mycustomclaim/myclaimsprovider", Value = "<NameOfTheClaimProviderTrust>");

When authentication through the first two Claims Providers, this claim is added to the claims I receive in my Relying Party Trust. For the third this claim is NOT added, even though I am successfully authenticated and my relying party trust accepts the authentication.

Can anybody tell me why this is? I have checked the ADFS eventlog, but there is no error. 

Can anybody give me a clue how I can debug this?

Is there any (undocumented) requirement that should be met before acceptance transform rules are fired?

Thank you in advance.