What behavior do you want your users to follow? How can you make that behavior more frequent?

The Fogg model may have the answer to this question.

The Fogg Behavior Model shows that three elements must converge at the same moment for a behavior to occur: Motivation, Ability, and a Trigger.

Motivation – what motivates your user to take this action? It was be any rewarding incentive that may encourage the user to take any sort of action. For more information on rewards I highly recommend Hooked by Nir Eyal.

Ability – this defines the ability of the user to take action that you want.

Trigger – some prompt must be present to remind the user to take the intended action. The triggers can be internal (boredom, fear etc) or external (push notifications)

So which one should try to iterate upon to make the behavior more frequent?

In my opinion, it is almost always easier to make the ability aspect of the Fogg model for the user to make the behavior more frequent. Make the intended action so easy that user almost has to put no effort into it.

Of course you can work on motivations and triggers but those needles are harder to move. However, ease of the action is totally within your control as you build your product.