Many Overground services have to share tracks with other services (for example between Liverpool Street and Clapton, or more particularly the freight trains on the West and South London lines.
You must have been very unlucky (or very selective) to have had to wait as much as twenty minutes for an Overground service, let alone more. Only the Emerson Park shuttle and the outer reaches of the Enfield/ Cheshunt routes (beyond their divergence at Edmonton) have a service less frequent than 3 trains per hour. Even then, with 2 tph the average time you would wait would be 15 minutes (maximum 30 minutes).
The busiest section is between Dalston Junction and Surrey Quays (up to 16 trains per hour, or better than every four minutes).