Why Eren can't fully transform?

Question

In season 3 episode 1 they show Eren can't transform properly, having shorter legs, less "meat" and not reaching 10 meters with his titan. Why is this so? Why can't Eren fully transform? AFAIR he never transformed half way before

Answer 1

This is expanded on more in the manga, in this instance he had been testing his abilities for hours and had transformed three times with his titan form being smaller and weaker each time and with little control over himself on the second attempt.

The third transformation resulted in the opening scene of the anime episode.