You can't offer them to some players because they probably have recently been asked already. There's a little period of time between you asking and if they accept or reject it, to where you an ask again.
And depending on the player and how you want to use him, you accordingly set the PPM.
eg. A player like Nasri who sometimes plays AML will cut inside so using the Place Shots PPM will see him score more (For me atleast).
Someone like Hulk, plays better with Shoot From A Distance/ Shoot with Power - As he has good technique and long shot attributes.