There could be many reasons resulting into this. For example, if the configuration and set up are not correct, then you may end up with no echo measurement. However, if you are sure that your configuration and test set up are correct, probably this is because the ERL (Echo Return Loss) is too high.
Abacus has a cut off ERL of 60 dB. If the measured ERL is equal to or larger than 60 dB, the system considers there is no echo. If this happens, you won't see that in the variance measurement from the system. Abacus has been designed in such a way that whenever there is no measurement, it will not be included in Variance.