But when a random no-name Titan ate Eren Jaeger... well that's when Eren Jaeger transformed into a Titan for the first time, inside the titan that ate him.
My question is... why didn't Marcel and why didn't Frieda transform after they were eaten?
Maybe Marcel had his legs eaten off, first (I don't remember), and his body was unable to transform because it was recovering, but then, when Reiner Braun had a sword through his neck he was able to transform into a Titan, no problem.
Similarly, Frieda might have been exhausted from her fight with Grisha, but when Eren was exhausted after training with his newly enhanced armor ability he was still able to transform, albeit to a smaller Titan.
Any ideas or is this a plot hole?