User blog comment:Yxz2014/Possible big plot hole??/@comment-16552509-20141001092724

At the time Eren's power as a titan shifter hasn't manifested yet - that's why, despite continuously hurting himself both in training and when he lost limbs to titans, he hasn't healed. We don't need evidence to know that Eren is someone who screams a lot, and yet only until much later in the story do we see his yelling being able to summon any nearby titans to a specific target.

The answer is that simple; his powers hadn't manifested yet, and so the titan that devoured him had not been able to turn back into a human.