From what I've read from NASA's Mars Design Reference Architecture (DRA) 5.0 and other articles, the use of ISRU for the ascent vehicle propellant is usually considered (be it atmospheric based, surface based or both). However, I haven't seen any mention as to producing fuel for the return trip.
Is there any major setback that doesn't allow for this? For instance, not having enough time to produce the amount of fuel needed or not enough resources. Sources are appreciated, if available.