Hey Mathias,
What is the actual device type you are talking about here?
From the sounds, it appears to be a Cisco device that may have a new/different OID perhaps?
What is the model type that it is being created as? There are a few filters for the Cisco based families. Model type being one of them, OID, etc.
What I have done in the past for another customer with a newer Cisco switch was utilize Model Type Editor to update the sysOIDVerifyList for the SwCiscoIOS model type to include the new OID.
Before the change it was GnSNMPDev or the generic Cisco model type that we have.
After the change it becamse SwCiscoIOS (remodel of course) and was then placed in the family.
You can try that if this is the same scenario you are seeing.
Thanks!
Matt