1. not be noticed or dealt with efficiently
we must prevent good candidates from slipping through the net
If someone 'falls through the net' or 'slips through the net' it means they have been missed, overlooked or let down by a system that was intended to help them. You can also use these expressions to talk about someone getting away with something when there is a system designed to catch you. Similarly, when you 'fall through the cracks' it means you have been overlooked or not been included with others. You can also say something 'fell through the cracks' when something was not done or completed. These are all idiomatic expressions used in both social and professional contexts.