There can be many, many reasons that this kind of failure would happen. Typical examples are mis-configuration in the settings, mistakes in the physical connection, and the failure in System Under Test (SUT). Users are advised to check all these to ensure the test set up is proper.
A very common mistake with many users is that the firmware of the Abacus is not compatible with the software in the Abacus GUI (PC). Users are advised to upgrade the firmware of the Abacus, otherwise this may result in unexpected system behavior.