Is the rule of not paying rent if you aren't asked for it, in the rules of Monopoly? Or is this something that people have made up? When is this ok?
According to the official rules, property owners must ask for the rent before the player after the next player rolls the dice or they forfeit the rent.
The owner may not collect the rent if they fail to ask for it before the second player following throws the dice.
So if the players are labelled A to D, and A should pay rent:
A ... B .x. C ... D
The lessor should request rent before C throws the dice, at the position marked X.