We have realized that transaction trace is nor working in two nodes of a cluster of 4.
If you try to trace traffic in those 2 nodes you don't get anything. If you trace using a filter with the URL of an application deployed in that cluster, you get traffic of 2 nodes (A and B) but nothing in the other 2 (C and D).
Collector having issues with traces database. Restarting EM or moving agent to another EM solves the issue.