Answer:
Average amplitude is the average magnitude in a wave cycle of all instant values. Average amplitude is the ratio of the sum of all instantaneous values to the number of instantaneous values considered in cycle.
When the average amplitude is non zero then the problem arises is it make it ineffective to carry a signal over the transmission medium.
An issue arises when there is a long 0s or 1s sequence and the voltage level is maintained for a long time at the same value. This creates a receiving end problem because the clock synchronization is now lost due to the lack of any transitions.