Without seeing the model, the only possible cause that comes to mind is that the information is stored on tokens which gets pushed to the list and enter a sink immediately after getting pulled. Whether copying labels from that token to the puller is successful would depend on which token arrived first. If the puller is waiting and then something is pushed, the puller continues first and can copy the labels. Otherwise the pulled token enters the sink before the puller continues.
If that is indeed the issue, add a Breathe activity after Push To List to always give the puller enough time to copy the labels.
