Which modules are shown in a flex relate field?

Hello!

I'm in SugarCRM CE 6.5.x.

As you may know, in the Calls module (could be any other) there is a flex relate field where the user can link a call record to any other record in the modules available in the flex relate field module list: Contacts, Accounts, etc. In that list I can find a couple of custom modules I once created. These modules have a one-to-many relationship with Calls module, so I understand this is the reason that makes these modules to be in the flex relate field module list for Calls. But I have a third custom module that has the same relationship with Calls and does NOT appear in the list. Any ideas of what can be causing this behaviour? Any place in the code where I can check if there is a missing file, a missing vardef, etc.?

Thank you very much for your help!

Jaume