[SR-Users] Transaction Lookup Failing

Hey all, Trying to debug an issue with canceling an invite. I have two different types of clients. On one client, canceling an invite works correctly. With the other client, it t_check_trans fails. Both clients show the same request/responses: On Client A: -> INVITE <- 183 -> PRACK <- 200 -> UPDATE (from successful STUN binding) <- 200 -> CANCEL <- 200 On Client B: -> INVITE <- 183 -> PRACK <- 200 -> UPDATE (from successful STUN binding) <- 200 -> CANCEL <- 481 (this is sent from my kamailio script when t_check_trans fails) I can't see much that's different between the leading requests/responses of each. They seem to be virtually identical (with the exception of IP addresses and tag values, of course). This makes sense, since they're both based on PJSIP (albeit different versions of PJSIP, but not off by much) Is there anything in particular I should be looking out for that might be obvious? I could provide more detailed logs and traces if required. Best, Colin