User blog comment:Yxz2014/Possible big plot hole??/@comment-9595671-20141023075243

I think the issue was that Eren was still alive in the Titan's stomach at the time. When Ymir ate Marcel, its possible she ate him in a way that instantly killed him and therefore prevented him from simply transforming inside her and escaping.

Basically I think a Titan only gains a Titan Shifter's powers when the shifter dies after they eat them, if the Shifter is still alive he/she still has an opportunity to escape.