According to my experiences, it is likely that your constraint does not consider all possible situations.
For example, if you would like to make a constraint that the delivery of Shipment A must be right after its pickup, then you need:
1) if currAct is the delivery of Shipment A, the constraint returns not fulfilled break if prevAct is not the pickup of Shipment A;
but this is not sufficient, and you need the following too (which is more likely to be missing):
2) if currAct is not the delivery of Shipment A, the constraint returns not fulfilled if prevAct is the pickup of Shipment A.
Hopefully my example is valid and reflects your case.