Seat selection only 90 days prior?

Just booked a return to LAS in October, and when I went to Manager my flights, it doesn't allow me to select a seat and says that's only available from 90 days before departure?
I thought you could do this more or less from when you book the flight - did something change or did I imagine I've been able to select a seat way ahead previously ?| ???
I thought you could do this more or less from when you book the flight - did something change or did I imagine I've been able to select a seat way ahead previously ?| ???