Since a plant’s goal is simply to reproduce itself, once it has produced mature flowers, it will often stop producing any more.
You can eliminate the signals telling the plant to stop flowering by regularly removing faded flowers. Petunias, cosmos, salvia and marigolds all will start blooming once again if old flowers are removed.
The process is called ‘deadheading’ and though it is a boring and repetitive job, your plants will blissfully bloom if you do it often.
Simply snap (or cut) off the flower just above the set of leaves growing below it.