Poor service

I have used Avis now twice via my insurance company and I had to wait so long until my vehicle got to me on both the occasions . booking was made a day in advance and upon arrival there was no car avail . I was asked to wait until a car was brought from their depo. On the 1st experience they over booked vehicles and was a forced to give me an upgrade . surely if a car is booked out to a customer,there shouldn't be delays on this nature.