Recurring problem: Keeping the player from thinking the game is endless.

The common technique for doing this is as follows

Give the player a goal. Players tend to keep playing (unless bored or frustrated) if an accepted goal is incomplete.
As one goal is complete, another goal becomes available.
Layer overlapping goals with a spectrum of completion times. No matter where the player is in the game, they always have open short term, medium term and long term goals.

A classic example is Civilization.

If players are no longer playing the issue is typically some variant of the following is at play.

Goals are being communicated poorly. There are million ways of fixing this, but in general be more explicit. If this is one of your first 10 games, attempt crisp clear player feedback. Designers need to master this basic skill before they attempt to be subtle and clever.
The game lacks systems for short term, medium term and long term goals. Goals are structured like rungs of a ladder. If the player cannot immediately reach out and grab the next rung, they will assume there is nothing above. A game with only a single long term goal will often be treated by the player as a rung too far.
The inner loop of your game isn't fun. All the goals in the world matter not a whit if the core interactions are draining the player's interest.

take care,
Danc.