What happened to Eren that made him unable to turn into a Titan in chapter 53 of the manga? I am referring to when he has trouble turning into full-sized Titan after he underwent intelligence and endurance testing. It happened in chapter 53 of the series, during the Uprising Arc. 

This is a quote from the Attack on Titan Wiki:


  During an experiment to test Eren's hardening abilities, a severed Titan form of Eren falls to the ground. Hange is unable to get a response from him and because Levi notices that this Titan is underdeveloped, Mikasa and Hange remove Eren's body from the Titan. [...]. A day later, Eren finally wakes up. He doesn't seem to remember anything from the experiment and is upset to find out that there were no signs of the hardening ability. Hange explains that since Eren transformed without being able to employ the hardening ability, they decided to run intelligence and endurance tests. [...]. However, after one hour of testing, Eren suddenly, after writing something about his father, became unable to perform the tasks and eventually left his Titan body. After thirty minutes of rest, he transformed again. However, this time his Titan form was only 13 m tall, contrary to the usual 15 m, and couldn't perform any of the intelligence tests. For the third time, the Titan was only 10 m tall and was incomplete, with his human body being attached to it more firmly than before. Eren takes this news as a proof that the operation to retake Wall Maria cannot be performed at the moment. 


Why did this happen and how was it resolved?
I'm pretty sure this is due to the fact that turning into a titan requires energy.  Serially turning into  one like that was wearing him down, such that he couldn't turn into the normal full-sized titan of normal titan intelligence.  If he burned enough energy as a titan or through regeneration, he would not be able to turn into a titan at all for a while, as established much later in the manga.